"/usr/src/kernel" is being hard-coded in multiple recipes so far, move its
definition to bitbake.conf.
(From OE-Core rev: eb9f900527e02ca08a1de14b4ac773f513bb1ee4)
Signed-off-by: Ming Liu <peter.x.liu@external.atlascopco.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>