main/postmarketos-mkinitfs: handle file system modules better on generic kernels

This MR makes postmarketos-mkinitfs work better on generic kernels (e.g. from Alpine) by making sure necessary file system modules are always included in the initramfs and loaded when necessary. See commit log for details.

This is a replacement for !3164 (closed), now actually tested on a device with FAT boot partition and QEMU with ext2 boot partition. :)