machine-overrides-extender.bbclass: Add filter out support
The allow easier enablement of mainline BSP support, we need to allow
for specific overrides to be filtered out, before and during the
extending process.
The new MACHINEOVERRIDES_EXTENDER_FILTER_OUT variable does exactly
this and will be used in a subsequent commit to enable the
'use-mainline-bsp' override.
Change-Id: Ieaec9d5eb27ccc73b7b0097cb365c519978bd080 Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
