classes/image: support package-management in IMAGE_FEATURES
Previously the "package-management" feature was only fully supported
when inheriting core-image.bbclass, which is not really ideal given that
this is the standard way of adding runtime packaging to an image in
OE-Core.
Part of the fix for [YOCTO #5424].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
