isequal: add --skip for both left and right file

isequal: New option --skip as a shortcut for --skipleft and --skipright
at once.

Use that for the .list-ref files, so they now contain the first line of
the output, too, which is ignored.

test/asm/listing/Makefile

@@ -106,7 +106,7 @@ ifneq ($(wildcard $1.list-ref),)
 #	we have a reference file, compare that, too
 
 #	remove first line which contains a version number
-	$(ISEQUAL) --skipright=1 $1.list-ref $$(@:.bin=.lst)
+	$(ISEQUAL) --skip=1 $1.list-ref $$(@:.bin=.lst)
 endif
 
 endef # LISTING_template