kernel-yocto: clean overly complex branch checkout

kernel-yocto: clean overly complex branch checkout

Since the git fetcher ensures that branches exist, we no longer need to
validate the branch and have a conditional checkout of the source.

We can remove some checks and ensure that whenever we exit the
do_kernel_checkout routine that a branch is always checked out.

(From OE-Core rev: 2ffa3f8be6996877cd552ff22260de35c19c413d)

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