binman: Give a sensible error if no command is given

At present if 'binman' is typed on the command line, a strange error about
a missing argument is displayed. Fix this.

These does not seem to be standard way to add the 'required' argument in
all recent Python versions, so set it manually.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/cmdline.py

@@ -37,6 +37,7 @@ controlled by a description in the board device tree.'''
         '3=info, 4=detail, 5=debug')
 
     subparsers = parser.add_subparsers(dest='cmd')
+    subparsers.required = True
 
     build_parser = subparsers.add_parser('build', help='Build firmware image')
     build_parser.add_argument('-a', '--entry-arg', type=str, action='append',