core-image-ptest: Switch to BBCLASSEXTEND parallel execution

core-image-ptest: Switch to BBCLASSEXTEND parallel execution

Replace the two images where tests are run serially with an image
per ptest in the form core-image-ptest-XXX using class extensions.

The original two images are then replaced with dependencies on these
images. This allows parallel execution of the tests.

The downside to this approach is the parsing time of an image generating
this number of class extensions is slow but making it easier to execute
and collect test data should outweigh that.

A useful advantage is that the dependencies of each ptest package are
checked/tested indvidually.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>