Before this commit an oem image still had the kiwi initrd_system
set as default. As we are decommission the custom kiwi initrd
concept the default should be changed. It is still possible to
use a custom kiwi initrd but it needs to be explicitly requested
via the initrd_system="kiwi" attribute. In addition to the
changed default a runtime check was introduced that checks
the presence of the boot= attribute which only makes sense
in combination with the kiwi initrd_system. If boot= is
set but initrd_system="kiwi" is not, a message is raised that
explains the situation and either requests setting initrd_system
properly or deleting the boot attribute. The change only
affects people who still use oem with a boot="oemboot/..."
setting and no explicit selection of kiwi as the initrd_system.
As these image type configurations should not be in use anyway
because this is all legacy and announced to go away, we
need to make the next step and enforce a new default in
code. This is related to Issue #1299
54ab18c96c