x86: OLPC: Hide OLPC_OPENFIRMWARE config switch

OLPC selects OLPC_OPENFIRMWARE unconditionally. If OLPC=n then
the OLPC_OPENFIRMWARE functionality is pointless.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andres Salomon <dilinger@queued.net>