libpayload: add support for vboot_handoff

The vboot_handoff structure needs to be parsed from the coreboot tables.
Add a placeholder in sysinfo as well as the ability to parse the
coreboot table entry concering the vboot_handoff structure.

Built with unified boot loader and ebuild changes. Can find and use
the VbInitParams for doing kernel selection.

Change-Id: If40a863b4a445fa5f7814325add03355fd0ac647
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/2720
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

payloads/libpayload/include/sysinfo.h

@@ -88,6 +88,8 @@ struct sysinfo_t {
 	struct cb_mainboard *mainboard;
 
 #ifdef CONFIG_CHROMEOS
+	void	*vboot_handoff;
+	u32	vboot_handoff_size;
 	void	*vdat_addr;
 	u32	vdat_size;
 #endif