x86: OLPC: Cleanup config maze completely

Neither CONFIG_OLPC_OPENFIRMWARE nor CONFIG_OLPC_OPENFIRMWARE_DT are
really necessary.

OLPC selects OLPC_OPENFIRMWARE unconditionally, so move the "select
OF" part under OLPC config option and fixup the dependencies in
Makefiles and code.

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