x86, olpc: Rework BIOS signature check

The XO-1.5 laptop is not currently detected as an OLPC machine because
it fails this XO-1-centric check.

Now that we have OLPC OFW support in the kernel, a more sensible
check is to see if we found OFW during boot and check the architecture
property.

Also remove a now-meaningless codepath, as we're always going to have
OFW support with OLPC.

Signed-off-by: Daniel Drake <dsd@laptop.org>
LKML-Reference: <20100923162846.D8D409D401B@zog.reactivated.net>
Cc: Andres Salomon <dilinger@queued.net>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>