bitbake: lib/bb/siggen: show a diff when dumping changes to multi-line values

When dumping changes to signatures e.g. output of bitbake -s printdiff,
if for example a function has changed, it's much more readable to see a
unified diff of the changes rather than just printing the old function
followed by the new function, so use difflib to do that.

Note: I elected to keep to one item in the returned list per change,
rather than one line per line of output, so that the caller can still
look at changes individually if needed. Thus I've added some handling to
bitbake-diffsigs to split the change into lines so that each line is
displayed indented.

(Bitbake rev: 4d254ae63a35231c98e3f73f669b040ed1144042)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/bin/bitbake-diffsigs

@@ -67,7 +67,9 @@ def find_compare_task(bbhandler, pn, taskname):
                 recout.append("Unable to find matching sigdata for %s with hash %s" % (key, hash2))
             else:
                 out2 = bb.siggen.compare_sigfiles(hashfiles[hash1], hashfiles[hash2], recursecb)
-                recout.extend(list('  ' + l for l in out2))
+                for change in out2:
+                    for line in change.splitlines():
+                        recout.append('  ' + line)
 
             return recout