fix Multiple .bb files ERROR

Hello,

I'm building images using the danny branch for both meta-raspberrypi and Yocto's Poky meta layers.

When building I'm having this error [1].

Removing 'PREFERRED_PROVIDER_virtual/libgl ?= "vc-graphics-hardfp' from meta-raspberrypi/conf/machine/include/rpi-default-providers.inc solves the issue and the build finish without errors.

So, I think that commit bff90c7 ("rpi-default-providers: Remove virtual/libgl provider") on master branch should also be added to the danny branch. The commit can't be cherry-picked cleanly so I added a new commit based on bff90c7.

Thanks a lot and best regards,
Javier

[1]
Build Configuration:
BB_VERSION        = "1.16.0"
TARGET_ARCH       = "arm"
TARGET_OS         = "linux-gnueabi"
MACHINE           = "raspberrypi"
DISTRO            = "poky"
DISTRO_VERSION    = "1.3"
TUNE_FEATURES     = "armv6 vfp arm1176jzfs"
TARGET_FPU        = "vfp"
meta              
meta-yocto        
meta-yocto-bsp    = "danny:2659f276cfb805c38f98f05fe90629ec7d9155eb"
meta-singularity-raspberrypi = "danny:55625c9aace99c7c07d5ef99308963728398ba6e"

NOTE: Resolving any missing task queue dependencies
NOTE: Preparing runqueue
ERROR: Multiple .bb files are due to be built which each provide virtual/libgles2 (/home/javier/collabora/devel/yocto/meta-raspberrypi/recipes-bcm/vc-graphics/vc-graphics-hardfp.bb /home/javier/collabora/devel/yocto/poky/meta/recipes-graphics/mesa/mesa-dri_8.0.4.bb).
 This usually means one provides something the other doesn't and should.
ERROR: Multiple .bb files are due to be built which each provide virtual/libgl (/home/javier/collabora/devel/yocto/meta-raspberrypi/recipes-bcm/vc-graphics/vc-graphics-hardfp.bb /home/javier/collabora/devel/yocto/poky/meta/recipes-graphics/mesa/mesa-dri_8.0.4.bb).
 This usually means one provides something the other doesn't and should.
ERROR: Multiple .bb files are due to be built which each provide virtual/egl (/home/javier/collabora/devel/yocto/meta-raspberrypi/recipes-bcm/vc-graphics/vc-graphics-hardfp.bb /home/javier/collabora/devel/yocto/poky/meta/recipes-graphics/mesa/mesa-dri_8.0.4.bb).
 This usually means one provides something the other doesn't and should.
NOTE: Executing SetScene Tasks
NOTE: Executing RunQueue Tasks