`postmarketos-mkinitfs` should fail hard, if it should generate something, but the dependency is missing

Created by: ollieparanoid

Fail hard, when:

• generate_bootimg is set, but mkbootimg is not installed or failed
• generate_legacy_uboot_initfs is set, but mkimage is not installed

It should print a meaningful error message, that explains how to fix this. Example:

ERROR: You have set "deviceinfo_generate_bootimg" to true, but the "mkbootimg" package is not installed.
In order to fix this, please add "mkbootimg" it to your device-package's "depends=" line.

... and after that bail out with exit 1 (so both apk and pmbootstrap will return an error).