bitbake: implement command to get all possible targets and their dependencies

bitbake: implement command to get all possible targets and their dependencies

Add a new command generateTargetsTree() which returns a dependency tree of
possible targets (tasks and recipes) as well as their dependency information.

Optional parameter 'klass' also ensures any recipes which inherit the
specified class path (i.e. 'classes/image.bbclass') are included in the model

Signed-off-by: Joshua Lock <josh@linux.intel.com>