so after a day of tinkering, building and compiling from source code I have finally managed to mount the
mmcblk0p10 3GB partition as an overlay extroot partition in OpenWRT 19.07-snapshot.
I have read the forum, specifically this thread , they were talking about adding
blockd packages to the build, it was supposed to solve the problem, when even after going through the official extroot tutorial, the partition wasn't mounted, logs saying fstools can't find fstab file. I am not sure if this solves the issue on 18.06, but it certainly doesn't work on 19.07.
What solved the issue was using patch from this bug report. I wasn't able to find much info about it, so I had to find which file to patch, it is located here:
/openwrt/build_dir/target-arm_cortex-a15+neon-vfpv4_musl_eabi/fstools... (the fstools folder can have different version in the name). I couldn't find any mention about it in the main thread which is now locked, so I hope it's okay to post this in the new one.
If anyone wants to build it themselves, it is ok to use the official extroot tutorial , but you have to mount the old overlay to
/rwm as the tutorial says.
My question is.. is there any chance this patch will be pulled into the 19.07 official branch, so in the future I can use the pre-built images to flash new versions? Even though I had a lot of "fun" compiling my build, it is rather time consuming.
Also if anyone is interested, I can provide the final sysupgrade image from the current 19.07-snapshot (as of today).