nvidia/cbootimage: update to master

It contains a number of fixes to bugs found
by Coverity Scan.

Change-Id: I362a069afd37783f59d8831e44ae885e8490819e
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/7392
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>