when enable the multiple workspace, the PLATFORM_DIR still is $(WORKSPACE)\AnyPkg, even though it is in a PACKAGES_PATH folder. this patch fix this issue to use the real path. 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>