sstatesig.py: Fix image regeneration issue

With the "ABI safe" recipes, we've been excluding those from signatures. This
is fine in the general case but in the specific case of image recipes it breaks.

A good test case is the interfaces file. Editting this causes init-ifupdown
to rebuild but not an image containing it (e.g. core-image-minimal).

We need to ensure the checksums are added to the image recipes and this change
does that.

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