vboot: Clean up vboot code

1. Remove unused functions/structures.
2. Add checks for NULL return values.
3. Change prefixes to vb2 instead of vboot for functions used internally
within vboot2/
4. Get rid of vboot_handoff.h file and move the structure definition to
vboot_common.h
5. Rename all functions using handoff structure to have prefix
vboot_handoff_*. All the handoff functions can be run _only_ after cbmem
is online.
6. Organize vboot_common.h content according to different
functionalities.

BUG=chrome-os-partner:55431

Change-Id: I4c07d50327d88cddbdfbb0b6f82c264e2b8620eb
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/15799
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

src/vendorcode/google/chromeos/gnvs.c

@@ -24,7 +24,7 @@
 #include "chromeos.h"
 #include "gnvs.h"
 #if CONFIG_VBOOT_VERIFY_FIRMWARE
-#include "vboot_handoff.h"
+#include "vboot_common.h"
 #include <vboot_struct.h>
 #endif