Various C projects provide a 'make help' target. Our root directory
does so. The roms/ directory lacks a such rule, but already displays
a help output when the default target is called.
Add a 'help' target aliased to the default one, to avoid:

  $ make -C roms help
  make: *** No rule to make target 'help'.  Stop.

Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190920171159.18633-1-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>