When integrate the latest UefiPayload with coreboot 4.14 or older, a
common error would occur like "E: Not a usable UEFI firmware volume."
The change adds an entry for how to solve this issue.
Cc: Guo Dong <guo.dong@intel.com> Reviewed-by: Ray Ni <ray.ni@intel.com> Signed-off-by: Aaron Li <aaron.li@intel.com>