BaseTools: check CONF_PATH env to get the configure files

BaseTools: check CONF_PATH env to get the configure files

Add CONF_PATH env check. First priority is user set the conf dir by
--conf option, then the CONF_PATH env, the last one is the standard
WORKSPACE(PACKAGE_PATH)/Conf.
Also print the conf path directory in the build log.

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>