bitbake: bitbake-layers: fix duplicated help info

There are two "help" lines when run bitbake help under python 2.6.6
(perhaps all python 2.6):

$ bitbake-layers help
[snip]
help
    display general help or help on a specified command
[snip]
help
    display general help or help on a specified command

This is because the cmd.py in python 2.6.6 also has a do_help() function
as our bitbake-layers, and the cmd.Cmd.get_names() doesn't consider the
overload, this has been fixed in python 2.7, use set() to remove the
duplicated would fix the problem.

[YOCTO #3428]

(Bitbake rev: 324ed96e28ec31cff8cef1824d20d40f9a5d46ad)

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

bitbake/bin/bitbake-layers

@@ -73,7 +73,7 @@ class Commands(cmd.Cmd):
         else:
             sys.stdout.write("usage: bitbake-layers <command> [arguments]\n\n")
             sys.stdout.write("Available commands:\n")
-            procnames = self.get_names()
+            procnames = list(set(self.get_names()))
             for procname in procnames:
                 if procname[:3] == 'do_':
                     sys.stdout.write("  %s\n" % procname[3:].replace('_', '-'))