bitbake: hob/bitbake: when an image is saved, it should require an image from layers

bitbake: hob/bitbake: when an image is saved, it should require an image from layers

Saving an image that requires another image will cause issues when
the second on is removed. So, we have agreed to "require" only the images
from layers.
The functionality is implemented in bitbake, in order to be more abstract,
and it is used by Hob when an image recipe is saved.

[YOCTO #4193]
(Bitbake rev: 28296ca78507ba2e414eb136c81afee65a8e25e5)

Signed-off-by: Cristiana Voicu <cristiana.voicu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>