RouterBOOT v7 on NOR devices no longer supports the old kernel ELF in YAFFS boot method. It needs a little extra packing into a (Mikrotik) NPK first.
I have rough tools to do this. They once worked for me, but usual disclaimers apply; expect device to catch fire:
- C program and example of using it alongside
fwtoolandyafutto repack a sysupgrade: https://github.com/john-tho/npk-packer - (Modified) Python library and script https://github.com/john-tho/npkpy/tree/create-npk and example of using it in build process: https://github.com/john-tho/openwrt/pull/7
The older Mikrotik devices also have issues with large initramfs images. These ramips devices will refuse to load an initramfs image with ELF entry point address > 0x80b81000: https://github.com/openwrt/openwrt/issues/13255#issuecomment-1772098866