yocto-bsp: use KBRANCH_DEFAULT in 'newbranch' cases

yocto-bsp: use KBRANCH_DEFAULT in 'newbranch' cases

The new yocto-kernel way of specifying that the branch sanity check
doesn't need to be run is to specify a default kbranch and build that.

In the case where we have a new kbranch but it's not yet in the repo,
we need to tell it that our new branch is the default and we're
building the default and that's how we avoid the sanity check and the
subsequent do_validate_branches() failure.

(From meta-yocto rev: dadd020c67e901609be3ac13a4ffd8cb28532966)

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>