kernel-yocto: move SRCREV validation to patching phase

kernel-yocto: move SRCREV validation to patching phase

Rather than attempting to condition the entire tree to machine SRCREV (since
we don't know what branch will be built), we can instead wait until patching
has completed and then confirm that we are indeed building a decendant of the
specified SRCREV. The result is a much simpler check, and no mangling of the
tree.

(From OE-Core rev: 97075af9e9a691276cd417f1181ca73223f52d1c)

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