image_types: Improve dependency handling between types (and use for sum.jffs2)

We're seeing a pattern of one image type needing to depend on another
type. A good example is jffs2 and sum.jffs2. This patch makes sum.jffs2
depend on jffs2 which will then allow a EXTRA_IMGAGECMD to be set for
sum.jffs2 individually without changing the jffs2 command. This allows the
-pad option to be configured differently.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>