main/postmarketos-mkinitfs: fix error on missing dtb
Currently if a non-existant dtb is defined in the deviceinfo file a broken error message is displayed:
==> kernel: device-tree blob operations
ERROR: File not found:
ERROR: postmarketos-mkinitfs-0.14.0-r1.trigger: script exited with error 1
This changes it into:
==> kernel: device-tree blob operations
ERROR: File not found: /usr/share/dtb/doesnotexist.dtb
ERROR: postmarketos-mkinitfs-0.14.0-r1.trigger: script exited with error 1