BaseTools: Skip module AutoGen by comparing timestamp.

[Introduction]

The BaseTool Build.py AutoGen parse INF meta-file and generate
AutoGen.c/AutoGen.h/makefile. When we only change .c .h code, the
AutoGen might be not necessary, but Build.py spend a lot of time on it.
There's a -u flag to skip all module's AutoGen. In my environment, it save
35%~50% of time in rebuild a ROM.
However, if user change one .INF meta-file, then -u flag is not available.

[Idea]

AutoGen can compare meta-file's timestamp and decide if the module's
AutoGen can be skipped. With this, when a module's INF is changed, we
only run this module's AutoGen, we don't need to run other module's.

[Implementation]

In the end of a module's AutoGen, we create a AutoGenTimeStamp.
The file save a file list that related to this module's AutoGen.
In other word, the file list in AutoGenTimeStamp is INPUT files of
module AutoGen, AutoGenTimeStamp file is OUTPUT.
During rebuild, we compare time stamp between INPUT and OUTPUT, and
decide if we can skip it.

Below is the Input/Output of a module's AutoGen.

[Input]
  1. All the DSC/DEC/FDF used by the platform.
  2. Macro and PCD defined by Build Options such as "build -D AAA=TRUE
     --pcd BbbPcd=0".
  3. INF file of a module.
  4. Source files of a module, list in [Sources] section of INF.
  5. All the library link by the module.
  6. All the .h files included by the module's sources.

[Output]
  AutoGen.c/AutoGen.h/makefile/AutoGenTimeStamp

[Testing]

This patch save my build time. When I make a change without touching
DSC/DEC/FDF, it is absolutely much faster than original rebuild,
35%~50% time saving in my environment
(compare to original tool rebuild time).
If I change any DSC/DEC/FDF, there's no performance improve, because it
can't skip any module's AutoGen.

Please note that if your environment will generate DSC/FDF during prebuild,
it will not skip any AutoGen because of DSC timestamp is changed. This will
require prebuild script not to update metafile when content is not changed.

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