linux-yocto: ensure that recipe specific defaults take precedence

In order to make KBRANCH assignments more flexible and easy to override the
default in each recipe as changed to a conditional assignment. But the
common include file, sets its own branch default to master.

This results in the recipes not overriding KBRANCH and master always being
used.

By putting the include after the default assignment, we now get the proper
default branch.

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