beaglebone.conf: Define policies for libgl* virtual providers

Fixes errors like

ERROR: Multiple .bb files are due to be built which each provide
virtual/mesa
(/home/kraj/work/angstrom-repo/sources/openembedded-core/meta/recipes-graphics/mesa/mesa_9.2.2.bb
/home/kraj/work/angstrom-repo/sources/openembedded-core/meta/recipes-graphics/mesa/mesa-gl_9.2.2.bb).

Signed-off-by: Khem Raj <raj.khem@gmail.com>