kernel-yocto: remove redundant SRCREV check

kernel-yocto: remove redundant SRCREV check

do_validate_branches checks to ensure that a valid machine SRCREV was
set. A test against an empty SRCREV is done in two separate locations,
we only need one, since the first check immediately returns and the
second check never hits.

At the same time, we can stop referring to the same commit hash by
3 different names. Instead we assign to a local variable at the
top of the routine, and refer to it at all times.

(From OE-Core rev: 05508339882c7cc1fe3f1f67f72314fdcab979b7)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>