Support for Mikrotik RB3011UiAS-RM?

What version of mikrotik you use?

MD5SUM from git pull is 93691d36a4175e2a94d07a83145ba807
I am using 6.42.1, but I can upgrade if needed

Upgrade to 6.44 https://download.mikrotik.com/routeros/6.44/routeros-arm-6.44.npk
I will now switch to its use too.

Ok, upgrading now

I update the git. Please reload and try again.

Hm, still the same

RouterBOOT booter 6.44

RBLHGG-60ad

CPU frequency: 716 MHz
  Memory size: 256 MiB
 Storage size:  16 MiB

Press any key within 2 seconds to enter setup..
trying dhcp protocol...... OK
resolved mac address C4:93:00:0F:30:DA
Gateway: 192.168.1.1
transfer started ............. transfer ok, time=1.21s
setting up elf image... OK
jumping to kernel code
Starting...
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
Starting services...
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory

I used Filezilla to upload OWL and OWL folder to /flash, is that an issue?

Try to edit OWL.sh via FTP and add this to the end of file:

/flash/rw/disk/OWL/bin/busybox

Doesnt help.

Illegal instruction

I tried to copy over binary generated by OpenWrt but script complains about it missing completely

I tested it on RB3011(RouterOS 6.43.12) and on RB450Rx4(RouterOS 6.44) and all works as expected...

Then I am doing something wrong, but I dont know what is wrong

Try to add to the end of OWL.sh:

/bin/busybox

and then:

/bin/busybox cat /proc/cpuinfo
RouterBOOT booter 6.44

RBLHGG-60ad

CPU frequency: 716 MHz
  Memory size: 256 MiB
 Storage size:  16 MiB

Press any key within 2 seconds to enter setup..
trying dhcp protocol...... OK
resolved mac address C4:93:00:0F:30:DA
Gateway: 192.168.1.1
transfer started ............. transfer ok, time=1.15s
setting up elf image... OK
jumping to kernel code
Starting...
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
BusyBox v1.00 (2019.01.31-10:03+0000) multi-call binary

Usage: busybox [function] [arguments]...
   or: [function] [arguments]...

        BusyBox is a multi-call binary that combines many common Unix
        utilities into a single executable.  Most people will create a
        link to busybox for each function they wish to use, and BusyBox
        will act like whatever it was invoked as.

Currently defined functions:
        [, ash, basename, bash, busybox, cat, chmod, chown, chroot, cp,
        dirname, echo, expr, find, hostname, ln, mkdir, mknod, mount,
        mv, rm, sh, test, touch, umount, uname, usleep

cat: /proc/cpuinfo: No such file or directory
Starting services...
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
BusyBox v1.00 (2019.01.31-10:03+0000) multi-call binary

Usage: busybox [function] [arguments]...
   or: [function] [arguments]...

        BusyBox is a multi-call binary that combines many common Unix
        utilities into a single executable.  Most people will create a
        link to busybox for each function they wish to use, and BusyBox
        will act like whatever it was invoked as.

Currently defined functions:
        [, ash, basename, bash, busybox, cat, chmod, chown, chroot, cp,
        dirname, echo, expr, find, hostname, ln, mkdir, mknod, mount,
        mv, rm, sh, test, touch, umount, uname, usleep

cat: /proc/cpuinfo: No such file or directory
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
BusyBox v1.00 (2019.01.31-10:03+0000) multi-call binary

Usage: busybox [function] [arguments]...
   or: [function] [arguments]...

        BusyBox is a multi-call binary that combines many common Unix
        utilities into a single executable.  Most people will create a
        link to busybox for each function they wish to use, and BusyBox
        will act like whatever it was invoked as.

Currently defined functions:
        [, ash, basename, bash, busybox, cat, chmod, chown, chroot, cp,
        dirname, echo, expr, find, hostname, ln, mkdir, mknod, mount,
        mv, rm, sh, test, touch, umount, uname, usleep

cat: /proc/cpuinfo: No such file or directory
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory

Now the same with:

/bin/busybox cat /proc/cpuinfo

I already added it in the last example as last line.
You can see Busybox complaining about /proc/cpuinfo missing

/bin/busybox cat /system/proc/cpuinfo
RouterBOOT booter 6.44

RBLHGG-60ad

CPU frequency: 716 MHz
  Memory size: 256 MiB
 Storage size:  16 MiB

Press any key within 2 seconds to enter setup..
trying dhcp protocol..... OK
resolved mac address C4:93:00:0F:30:DA
Gateway: 192.168.1.1
transfer started ............. transfer ok, time=1.26s
setting up elf image... OK
jumping to kernel code
Starting...
Initializing busybox
Illegal instruction
Launching telnetd
chroot: cannot execute telnetd: No such file or directory
Processor       : ARMv7 Processor rev 5 (v7l)
processor       : 0
BogoMIPS        : 1430.32

processor       : 1
BogoMIPS        : 1430.32

processor       : 2
BogoMIPS        : 1430.32

processor       : 3
BogoMIPS        : 1430.32

Features        : swp half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt 
CPU implementer : 0x41
CPU architecture: 7
CPU variant     : 0x0
CPU part        : 0xc07
CPU revision    : 5

I really cant understand why busybox generated for ARM32 cant run

Try this one: http://hosting.yapic.net/openwrt/ipq/busybox

My /proc/cpuinfo for RB450Gx4 is absolutely equal to Your.
We both use RouterOS 6.44.
It seems that the busybox binary is damaged when uploading via FTP.
For FTP i use Linux MC(Midnight Commander).

Yes since the CPUs are absolutely the same, but for some weird reason it does not work.
In order to enable devel account which drops you to root shell mkdir /pckg/option should work but it does not as the folder is not mounted

Yes. It is simply not possible that it does not work and yet it does not work!