Help on compiling openwrt-openfortivpn on Rpi 3B

Dear all

I just installed OpenWrt on a Raspberry pi 3 ver B. My idea was to use it as a VPN router connected to my company network, The VPN server of my company is a Fortigate and after some googling I've seen that there is an open source client compatible with Forticlient, called "openfortivpn". This package is not in the list of OpenWrt software, so I'm trying to compile it by myself.

I referred to this page: https://github.com/excelwang/openwrt-openfortivpn

which tells to download and unpack the SDK (I downloaded it from here https://archive.openwrt.org/snapshots/trunk/brcm2708/generic/)

then issue:

./scripts/feeds update base

I did this on a Red Hat 8 server. This command gives me a lot of errors. Here is the output:

$ ./scripts/feeds update base
Updating feed 'base' from 'git://github.com/openwrt/openwrt.git' ...
Cloning into './feeds/base'...
remote: Enumerating objects: 9068, done.
[.....]
Create index file './feeds/base.index'
ERROR: please fix feeds/base/package/Makefile - see logs/feeds/base/package/dump.txt for details
ERROR: please fix feeds/base/package/boot/uboot-at91/Makefile - see logs/feeds/base/package/boot/uboot-at91/dump.txt for details
[... same error for uboot-fritz4040, imx6, kirkwood etc]

$ cat logs/feeds/base/package/dump.txt
Makefile:11: /rootfs.mk: No such file or directory
gmake[1]: *** No rule to make target '/rootfs.mk'.  Stop.

$ cat logs/feeds/base/package/boot/uboot-at91/dump.txt
Makefile:17: /home/mario/OpenWrt-SDK-brcm2708-bcm2708_gcc-5.3.0_musl-1.1.16_eabi.Linux-x86_64/include/u-boot.mk: No such file or directory
gmake[1]: *** No rule to make target '/home/mario/OpenWrt-SDK-brcm2708-bcm2708_gcc-5.3.0_musl-1.1.16_eabi.Linux-x86_64/include/u-boot.mk'.  Stop.

The required files are not in the include folder but:

$ find . -name rootfs.mk
./feeds/base/include/rootfs.mk

I symlinked the files from there:

$ cd include
$ ln -s ../feeds/base/include/rootfs.mk rootfs.mk
$ ln -s ../feeds/base/include/u-boot.mk u-boot.mk

Now I got only two errors:

ERROR: please fix feeds/base/package/Makefile - see logs/feeds/base/package/dump.txt for details
ERROR: please fix feeds/base/target/imagebuilder/Makefile - see logs/feeds/base/target/imagebuilder/dump.txt for details

$ cat logs/feeds/base/package/dump.txt
Makefile:11: /rootfs.mk: No such file or directory
gmake[1]: *** No rule to make target '/rootfs.mk'.  Stop.

$ cat logs/feeds/base/target/imagebuilder/dump.txt
gmake[1]: *** No rule to make target 'dumpinfo', needed by 'all'.  Stop.

I don't know if symlinking was the right way, but reduced the errors from 20 to 2 but now I don't know how to proceed.
Thanks for your precious help.

Mario.