main/postmarketos-mkinitfs: upgrade to 2.5.0

• Add support for systems with merged /usr (MR 39)
• Improve support for deviceinfo format (MR 52)
• Add support for systemd (MR 51)
• Document archive compression formats/levels (MR 50)

When this is merged, the following commit can be dropped from the master_staging_systemd branch:

• 400ee96e: TEMP: main/postmarketos-mkinitfs: usrmerge fork

Tested on:

• lenovo 21bx (openrc)
• qemu-amd64 (systemd)
• oneplus-enchilada (openrc)