systemd/postmarketos-base-systemd: put libkmod in initfs-extra

It is needed by udev, which is in initfs-extra. If libkmod and its dependencies are put in the first initfs then the boot.img size increases by roughly 2 MB, which is a problem for devices with small boot partitions.

Tested on samsung-m0 with phosh. Kernel and initfs'es load just fine, and I was able to ssh into the device and check journalctl logs. Phosh never came up though, but I think that is a separate issue.

Fixes: #3510 (closed)