image_types: Ensure duplicate compressed types aren't created

This allows IMAGE_FSTYPES = "tar.bz2 tar.bz2" to work (and now
is faster since it will only do it once).

(From OE-Core rev: b24d4bcde21b013e48eaffea5f3a70d8b1df1047)

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

meta/classes/image_types.bbclass

@@ -17,7 +17,8 @@ def get_imagecmds(d):
                     types.append(basetype)
                 if basetype not in cimages:
                     cimages[basetype] = []
-                cimages[basetype].append(ctype)
+                if ctype not in cimages[basetype]:
+                    cimages[basetype].append(ctype)
                 break
         if not basetype and type not in types:
             types.append(type)