bitbake: bitbake-diffsigs: fix traceback with no arguments

In the move over to argparse we've made the two signature file arguments
optional and thus if -t is not in use we need to explicitly check if at
least one signature file has been specified - and if not, show an error
and the command-line help.

(Bitbake rev: 9011366689b26305281fcb2d412dcacece917e18)

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

bitbake/bin/bitbake-diffsigs

@@ -147,6 +147,10 @@ else:
             output = bb.siggen.compare_sigfiles(options.sigdatafile1, options.sigdatafile2, color=color)
         elif options.sigdatafile1:
             output = bb.siggen.dump_sigfile(options.sigdatafile1)
+        else:
+            logger.error('Must specify signature file(s) or -t/--task')
+            parser.print_help()
+            sys.exit(1)
     except IOError as e:
         logger.error(str(e))
         sys.exit(1)