BaseTools/GenFds: enhance to get TOOL_CHAIN_TAG and TARGET value

BaseTools/GenFds: enhance to get TOOL_CHAIN_TAG and TARGET value

when user don't set TOOL_CHAIN_TAG and TARGET by –D Flag, then GenFds
would report failure for format:
FILE DATA = $(OUTPUT_DIRECTORY)/$(TARGET)_$(TOOL_CHAIN_TAG)/testfile
so this patch enhance to get the TOOL_CHAIN_TAG and TARGET value by
following priority (high to low): 1. the Macro value set by -D Flag;
2. Get the value by the -t/-b option. 3. get the value from target.txt
file. Besides, this patch also remove the error checking for missing
-t/-b option.

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>