BaseTools: Add support to merge Prebuild and Postbuild into build Process

This feature is enhance build tool to incorporate execution of prebuild
and postbuild.

1.Prebuild script
a.DEFINE PREBUILD in DSC [Defines] section
b.Build command -D PREBUILD to override the one in DSC [Defines] section
1)If PREBUILD is a file, then this file will be used as prebuild script.
2)If PREBUILD is empty, then prebuild script will be disabled.
3)If PREBUILD is not defined in [Defines] section and not passed in on
command line, then prebuild script is also disabled.

2.Prebuild option
a.All options of build tool
b.TARGET, ARCH and TOOL_CHAIN_TAG value, Those value will be from
target.txt file if they are not in build command line.
c.Additional options following prebuild definition. Quotes are needed
when these additional options are present.
d.Quotes would also be required if the path to the prebuild command
contains space or special characters.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>