abl: add recipe to build (unsigned) ABL binary

[lumag]

Add a recipe to build an unsigned ABL binary. This can be useful to test the ABL changes. Currently this recipe uses external Clang binary, but it might be later migrated to meta-clang. The ABL build system depend on Python2 being present, thus the recipe will not be enabled if meta-python2 is not present.

Signed-off-by: Dmitry Baryshkov [email protected]