I've read about some of the ar71xx issues and porting it to ath97, but I am not sure whether this answers my question as well - please forgive me, I am a noob on non-x86 hardware.
The mr-3420 build has been pretty difficult up until 17.x as space is really limited. I think this might have been the reason why its support got removed in 18.x. However, tweaking packages here and there, I have always been able to make it boot with usb support and outsource the rest.
I tried to do the same with 18.x
I re-added the target/profile in the make files and it seems the rootfs gets built properly. However, the kernel is missing:
make: *** No rule to make target '/mnt/scratch/openwrt-imagebuilder-18.06.1-ar71xx-generic.Linux-x86_64/build_dir/target-mips_24kc_musl/linux-ar71xx_generic/tl-mr3420-v2-kernel.bin', needed by '/mnt/scratch/openwrt-imagebuilder-18.06.1-ar71xx-generic.Linux-x86_64/build_dir/target-mips_24kc_musl/linux-ar71xx_generic/tmp/openwrt-18.06.1-ar71xx-generic-tl-mr3420-v2-squashfs-sysupgrade.bin'. Stop.
Should I invest time in building the kernel myself (bonus question - is there a quick way/howto for just compiling the default kernel with default options for this target?) or will there be other, more grave issues kernel-wise?
Adding the average kernel size of 1,4M to my rootfs, I get 3409K which should totally be possible to fit on the router.
Thanks for any input and sorry again if I should just accept ar71xx is more or less considered dead.