classes/externalsrc: enable global inherit and simplify usage

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.

(From OE-Core rev: b1da10579a28f9a5260a0678f6f15ce4b5a2706c)

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