Ensure there is default command available if FETCHCMD_p4 isn't set.
Also clean up the getVar references whilst here.
(Bitbake rev: d150226d11d5f041f78c8c3ce4abc5465dbc81d8)
(Bitbake rev: d865692e8b76a1da80c80788946978f150f2c34b)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>