Fill out ChromeOS specific coreboot table extensions

ChromeOS uses two extensions to the coreboot table:
- ChromeOS specific GPIO description for onboard switches
- position of verified boot area in nvram

Change-Id: I8c389feec54c00faf2770aafbfd2223ac9da1362
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/866
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>