linux-yocto: make KBRANCH the exception and not the rule

linux-yocto: make KBRANCH the exception and not the rule

The kernel branch is no longer required by the yocto-kern-tools
to locate BSP feature descriptions (it is the MACHINE:KTYPE
descriptor), so we no longer require that the BSP branch be
explicitly set.

If a kernel branch is explicitly set, it is now used to trigger
a checks to ensure that the branch really is being built.
Otherwise the branch that the machine description creates will
be built (just as it always was).

This further simplies the use and configuration of a linux-yocto
based kernel recipe.

(From OE-Core rev: 3cac3ce65abae9dc253641a2004440a2b38fd44d)

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