mesa: Define broadcom packageconfig knob

Fixes
ERROR: mesa-2_21.3.0-r0 do_configure: QA Issue: mesa: invalid PACKAGECONFIG: broadcom [invalid-packageconfig]

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>