mbed: config: print detailed help in subcommand

Print detailed usage when unsupported arguments are supplied
with mbed config subcommand

Author: Naveen Kaje

mbed/mbed.py

@@ -3250,7 +3250,9 @@ def config_(var=None, value=None, global_cfg=False, unset=False, list_config=Fal
                 else:
                     value = program.get_cfg(var)
                     action(('%s' % value) if value else 'No default %s set in program "%s"' % (name, program.name))
+                    subcommands['config'].print_help()
     else:
+        subcommands['config'].print_help()
         subcommands['config'].error("too few arguments")