mesa: assign S in include file

mesa: assign S in include file

Anything including mesa.inc will have the sources extracted in
${UNPACKDIR}/mesa-${PV}.

The default for S is ${UNPACKDIR}/${BP}. ${BP} is ${BPN}-${PV}. Because
mesa.bb is named mesa, BPN will be mesa and thus S wasn't required for
mesa.bb but only for mesa-gl.bb. This also explains why this change is
fine for mesa.bb as the value of S won't have changed, the ${BPN} part
is now just hardcoded to "mesa" for mesa.bb instead.

No intended change in behavior.

Signed-off-by: Ross Burton <ross.burton@arm.com>
[added commit log and title]
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>