mainboard: Remove last_boot NVRAM option

The last_boot NVRAM option was deprecated and removed in
commit 3bfd7cc6.  Remove the last_boot option from all
affected mainboards to eliminate user confusion.

Change-Id: I7e201b9cf21dfe5dda156785bad078524098626d
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: http://review.coreboot.org/12316
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins)
206 files changed