x86: OLPC: Remove extra OLPC_OPENFIRMWARE_DT indirection

OLPC_OPENFIRMWARE_DT is just there to be selected by OLPC and selects
OF_PROMTREE. So let OLPC select OF_PROMTREE and remove that extra
config indirection. Fixup code and Makefile and use CONFIG_OF_PROMTREE
instead.

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