Draft: main/postmarketos-initramfs: do not re-mount boot partition into sysroot

Since quite a while we use UUIDs and not just labels in /etc/fstab, making this unnecessary. Instead of re-implementing the /etc/fstab feature, let the rootfs do its thing.

This has actually hidden a serious bug that /boot from /etc/fstab could not be mounted: https://gitlab.com/postmarketOS/pmbootstrap/-/merge_requests/2304 I guess the main question is how to transition current installations with fat32 or fat16 in /boot where /etc/fstab cannot mount it.