detect multiple installs

The postmarketOS rootfs can be flashed to various partitions on a target device, and to SD cards. It can happen, that the rootfs was flashed twice to different partitions, and then the initramfs will try to boot the first one that was found.

If you forgot about your other installation, a great deal of time can be wasted until you figure out that you aren't booting what you have just flashed but some old install instead.

@MartijnBraam proposed:

maybe its easy to make the initfs throw a warning when multiple partitions with the pmos_root label are found

I think this is a great idea. However, it might be that even if the rootfs was flashed only once, it may be found multiple times after running kpartx to find the partitions like we are doing it in the initramfs. So this needs to be tested carefully, with and without encryption and with internal storage as well as SD cards.