Build spec mentions $(FAMILY) macro be used in DSC/FDF to specify the tool
chain family, like GCC, MSFT. This patch add the support for this macro.
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>