main/postmarketos-mkinitfs: don't rm vmlinuz-*

Merged Oliver Smith requested to merge mkinitfs-fix into master
Adjust post-upgrade script to not remove vmlinuz-*. This causes problems
with devices using kernels from Alpine, where the kernel is not
installed to /boot/vmlinuz, as it is now the case in postmarketOS, but
e.g. in /boot/vmlinuz-rpi. The intention was to clean up files when
migrating from the old postmarketos-mkinitfs to the new one (>= 1.0.0).
However, the /boot/vmlinuz* files are managed by apk, so they should
already get removed.

Add -v, so it prints a message when removing a file from /boot.
Edited by Oliver Smith