toaster.bbclass: reinstate scan for artifacts in the sdk directory
During refactoring of the SDK/artifact scan code in toaster.bbclass,
the code to find other non-image artifacts in the images/ directory
was incorrectly removed.
Reinstate that code and clean it up so it's clearer what's happening
and so that non-image artifacts are correctly reported.
[YOCTO #8956]
Signed-off-by: Elliot Smith <elliot.smith@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
