gm.rb: make --cmock_option_cfg param optional

The address of the cfg.yml file in unit_test/ is
provided as the default value for this flag. This
enables a more ergonomic use of gm.rb, reducing
boilerplate arguments, for example:

gm.rb -m mocks/ -f file_to_mock.h

Signed-off-by: Brett Warren <brett.warren@arm.com>
Change-Id: I1d772f741e40663ca31a04234437c4b46444e9d1