scripts: ensure not specifying subcommand shows help text

scripts: ensure not specifying subcommand shows help text

With Python 2, argparse subparsers behaviour in Python 2 was to print
the usage information if the subparsers argument wasn't specified.
However, with Python 3.2.3 and later a subparsers argument is not
required by default, leading to errors when no arguments are specified:

  AttributeError: 'Namespace' object has no attribute 'func'

Restore the previous desired behaviour of showing the help text for
devtool, recipetool and the devtool-stress script by setting
subparsers.required to True.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>