Enhance move_to_dropins script
Allow any given feature group to be broken out into dropins by passing a
list of feature ids into the script as a third parameter.

Change-Id: Ie398bfb1e46c895bbbc56bd0db186cda3f17e2fe
Signed-off-by: Mat Booth <mat.booth@redhat.com>
Reviewed-on: https://git.eclipse.org/r/29385
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>
Tested-by: Alexander Kurtakov <akurtako@redhat.com>