mesa: add opencl DISTRO_FEATURE

mesa: add opencl DISTRO_FEATURE

OpenCL is an important part of the GPU-related world. It makes sense to
be able to provide headless environment (without OpenGL and Vulkan), but
having just OpenCL as a GPU user. Currently it is not possible since
mesa requires either of those to be enabled for the DISTRO.

Add new 'opencl' DISTRO_FEATURE, controlling enablement of OpenCL.

Note: Mesa, if built with the libclc packageconfig (which is required
for OpenCL driver) depends on the mesa-clc tool from the mesa-native
package. It is required to propagate opencl DISTRO_FEATURE to the native
set in order to be able to fulfill mesa -> mesa-native dependency as
otherwise mesa-native package will be skipped.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>