BaseTools: add some comment for .PrebuildEnv file's usage

This patch add some comments to explain why we use .PrebuildEnv file to
save environment variable settings set by the prebuild script.

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>

BaseTools/Source/Python/build/build.py

@@ -1029,6 +1029,11 @@ class Build():
         if self.Prebuild:
             EdkLogger.info("\n- Prebuild Start -\n")
             self.LaunchPrebuildFlag = True
+            #
+            # The purpose of .PrebuildEnv file is capture environment variable settings set by the prebuild script
+            # and preserve them for the rest of the main build step, because the child process environment will
+            # evaporate as soon as it exits, we cannot get it in build step.
+            #
             PrebuildEnvFile = os.path.join(GlobalData.gConfDirectory,'.cache','.PrebuildEnv')
             if os.path.isfile(PrebuildEnvFile):
                 os.remove(PrebuildEnvFile)