Please delete this thread.
Try removing/replacing systemd.... or triple check your dev mounts etc....
There are some alternatives which evolve making the kernel more debian-like.... but best to cut back the container first......
i.e.;
- Check this re: devtmpfs https://openwrt.org/docs/guide-developer/building_kernels_for_debian_binaries
From memory... there is a few things that need to be tweaked on the network side..... So best to disable networking on the guest/container whilst your testing the rest of the setup.....
Update us on how you go..... and someone should be able to help with the network side of things.....
Next to impossible 
The link I posted earlier has instructions for embedding /proc/config.gz
Openwrt is not debian. The dev and init subsystems in particular operate totally differently. Armbian, does not have this problem....
Otherwise.... if you need and easy option.....
- raw chroots work quite nice......
- go with Armbian
I'd be surprised of you can't find veth in the buildroot even with sunxi....
Have you tried hitting "/" and searching for veth?
These
CONFIG_FHANDLE: missing
CONFIG_EVENTFD: enabled
CONFIG_EPOLL: enabled
CONFIG_UNIX_DIAG: missing
CONFIG_INET_DIAG: missing
CONFIG_PACKET_DIAG: missing
CONFIG_NETLINK_DIAG: missing
Would be worth a look in > make kernel_menuconfig .... have you searched there?