Stop installing an empty /etc/fstab, and migrate old installations that have one

Describe your issue

What's the expected behaviour?

/etc/fstab contains the / and /boot mountpoints, and mount -a can mount them. Then we can stop mounting /boot in the initramfs and reduce a bit the complexity and code duplication.

What's the current behaviour?

Installations after https://gitlab.com/postmarketOS/pmbootstrap/-/commit/46789ccee8c75eade289359d2eeba212fe58a800 have those entries in /etc/fstab, but due to the lack of previous support in pmbootstrap, and #664 (closed) previous installations don't generate those files, and instead keep the default empty /etc/fstab file shipped. These installations cannot mount /boot from the rootfs, which means we have to keep code in the initramfs to re-mount /boot in the rootfs (after it's been mounted in the initramfs itself to read initramfs-extra)

How to reproduce your issue?

Should be possible to reproduce with pmbootstrap from git previous to the linked commit, and 23.06 branch of pmaports, though it's not tested. @Arnavion has one of those installs

CC @jenneron @Arnavion I won't have time to fix this soon, but adding an issue so we don't forget and can point to it from the initramfs