raspberry pi{3,4}: fix boot
Move our custom config.txt stuff to usercfg.txt. config.txt is replaced (by alpine's initramfs-generator I guess), so the stuff there does not have an effect.
We set the initramfs variable to initramfs
in config.txt. Boot currently fails due to it looking for initramfs-rpi
instead.
https://gitlab.com/postmarketOS/pmaports/-/merge_requests/2731 fixed or tried to fix the same issue.
Fixes https://gitlab.com/postmarketOS/pmaports/-/issues/2471.