conf/bitbake.conf: Add MACHINEOVERRIDES variable

conf/bitbake.conf: Add MACHINEOVERRIDES variable

By defualt it points to $MACHINE but sometimes its desired
to have more than one overrides stemming out of a machine
then they can be added to MACHINEOVERRIDES.

e.g. MACHINEOVERRIDES = "${MACHINE}:nslu2"

Note that if you redefine MACHINEOVERRIDES then default
override for machine has to be added to it explicitly
otherwise it will get lost.

Signed-off-by: Khem Raj <raj.khem@gmail.com>