conf/machine: Don't poke around providers which aren't machine specific/safe
Machines shouldn't be poking around PREFERRED_PROVIDERS which aren't
machine specific or at least machine safe. Kernels are machine specific
and the xserver is selectable. libx11 and mesa are now really a distro choice
and machine configurations shouldn't be poking around them as it just leads
to corruption, conflicts and confusion.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
