Update Makefile to support FFS file generation with new build option --genfds-multi-thread. Cc: Liming Gao <liming.gao@intel.com> Contributed-under: TianoCore Contribution Agreement 1.1 Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com> Signed-off-by: Yunhua Feng <yunhuax.feng@intel.com> Reviewed-by: Liming Gao <liming.gao@intel.com>