yocto-bsp: Add branch to SRC_URI for custom kernels

yocto-bsp: Add branch to SRC_URI for custom kernels

Without 'branch' in the SRC_URI, a SRCREV specified for a non-master
KBRANCH will result in a fetch failure since the branch tested by the
fetcher will default to master, which doesn't contain the SRCREV.
This fixes the problem by adding branch=KBRANCH to the SRC_URI.

Fixes [Yocto #6518].

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