local.conf: Make virtuals as fallbacks
local.conf: Make virtuals as fallbacks

Some machines like rpi use different graphics display drivers
e.g. vc4graphics and userland and it depends on machine feature
which one to use, default it userland, which needs mesa-gl to
provide libgl and vc4graphics uses mesa.

Turn this setting into a fallback in local.conf instead of
overriding it

Signed-off-by: Khem Raj <729d64b6f67515e258459a5f6d20ec88b2caf8df@gmail.com>

cbrake/oe-build,kraj/oe-build