Fixes related to /etc/fstab and mounting /boot in the initramfs

pmaports side of pmbootstrap!2304 (merged)

After quite some back and forth (including closing !5083 (closed)), came to the conclusion that what we want is:

• To mount /boot in the initramfs, and not in the rootfs, as we don't want to read /etc/fstab, and OpenRC localmount will not mount vfat or ext2 filesystems not all installations have proper /etc/fstab. See #2800
• To add as many sensible options as possible, identical to !5083 (closed). Unfortunately the previous point makes the duplication necessary
• We want to fix fstab when possible, to make things consistent