baremetal-image.bbclass: Emulate image.bbclass to handle new classes scope

baremetal-image.bbclass: Emulate image.bbclass to handle new classes scope

The new classes scope implemented on commit 7bd328f9d
made testimage.bbclass (and perhaps others) stop working
for baremetal-images, the expected way to run testimage
is no longer to use INHERIT but to use IMAGE_CLASSES
instead, however this functionality was not implemented in the
baremetal-image class until now.

Emulate image.bbclass allowing the baremetal-image class to
use IMAGE_CLASSES to fix this issue.

Set defaults for IMAGE_FEATURES to allow bitbake checks to
pass properly.

Signed-off-by: Alejandro Enedino Hernandez Samaniego <alejandro@enedino.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>