staging: olpc_dcon: add config options for XO_1 and XO_1_5, drop hardcoded XO-1 stuff

This adds CONFIG_FB_OLPC_DCON_1 and CONFIG_FB_OLPC_DCON_1_5 options for
allowing selection of XO-1 and/or XO-1.5 DCON support.  In the process,
it also forces the xo_1.c and xo_1_5.c files to build as separate units,
correctly selects between XO-1 and XO-1.5 at runtime, and adds some
hacks to allow xo_1_5.c to build.

This isn't the cleanest patch, but it'll get better as more global
variables are dropped.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>