bitbake: bitbake: update help message for dump-signatures

The bitbake -S had been updated which always reuires an argument, so
update the help info:
- Add the two args in the help message: none and printdiff

- Use type="choice" so that we can get more friendly error messages,
  for example:
  bitbake: error: option -S: invalid choice: 'printdiffX' (choose from 'none', 'printdiff')

- dump-signatures=DUMP_SIGNATURES -> dump-signatures=SIGNATURE_HANDLER

(Bitbake rev: 021b778fa4685bdde39e1a0f6c7c57632dcf792a)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/bin/bitbake

@@ -139,8 +139,8 @@ class BitBakeConfigParameters(cookerdata.ConfigParameters):
         parser.add_option("-n", "--dry-run", help = "Don't execute, just go through the motions.",
                    action = "store_true", dest = "dry_run", default = False)
 
-        parser.add_option("-S", "--dump-signatures", help = "Dump out the signature construction information, with no task execution. Parameters are passed to the signature handling code, use 'none' if no specific handler is required.",
-                   action = "append", dest = "dump_signatures", default = [])
+        parser.add_option("-S", "--dump-signatures", help = "Dump out the signature construction information, with no task execution. The SIGNATURE_HANDLER parameter is passed to the handler. Two common values are none and printdiff but the handler may define more/less. none means only dump the signature, printdiff means compare the dumped signature with the cached one.",
+                   action = "append", dest = "dump_signatures", default = [], type="choice", choices=("none", "printdiff"), metavar="SIGNATURE_HANDLER")
 
         parser.add_option("-p", "--parse-only", help = "Quit after parsing the BB recipes.",
                    action = "store_true", dest = "parse_only", default = False)