classes/externalsrc: enable global inherit and simplify usage
This class can now be inherited globally using INHERIT += rather than
needing to inherit it in the recipe itself. Additionally, instead of
setting S (and optionally B), set EXTERNALSRC (and optionally
EXTERNALSRC_BUILD) to point to the external source and build locations
respectively.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
