main/postmarketos-mkinitfs: fix trigger script

Adjust the trigger script to only check for the new deviceinfo path. The
purpose of this check is to make sure that a full device package is
installed, and only in that case run mkinitfs. Otherwise, it will fail
because e.g. no kernels may be installed.

With recent changes, /etc/deviceinfo is a part of devicepkg-utils and
only /usr/share/deviceinfo/deviceinfo really indicates that a device
package is installed.

Remove the "exit 0" at the end while at it, it did not do anything since
the script runs with "/bin/sh -e". Add a comment to make this clear.

Fix https://gitlab.com/postmarketOS/pmbootstrap/-/issues/2266