mkarchiso exits with non-zero exit code

Due to changes introduced in 57d510fe mkarchiso's _set_overrides() may evaluate to false if no override is specified and will exit mkarchiso with a non-zero exit code.

In the same commit changes were introduced to _prepare_airootfs_image() that may evaluate to false if no gpg key is specified and will exit mkarchiso with a non-zero exit code as well.