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

[Yonghong Zhu <yonghong.zhu@intel.com>]

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>
---
 BaseTools/Source/Python/build/build.py | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/BaseTools/Source/Python/build/build.py b/BaseTools/Source/Python/build/build.py
index f7634fa..4d05ee2 100644
--- a/BaseTools/Source/Python/build/build.py
+++ b/BaseTools/Source/Python/build/build.py
@@ -1027,10 +1027,15 @@ class Build():
 
     def LaunchPrebuild(self):
         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)
             if os.path.isfile(self.PlatformBuildPath):
                 os.remove(self.PlatformBuildPath)
-- 
2.6.1.windows.1