oeqa/controllers/masterimage: add a base class for hw targets

oeqa/controllers/masterimage: add a base class for hw targets

Right now GummibootTarget is the only hardware TEST_TARGET with deployment,
but we will add more, so let's make an abstract base class, that will
do the common thing for all the hw targets.

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>