DuetPkg: Add POSTBUILD in DSC files to run post-build automatically

DuetPkg: Add POSTBUILD in DSC files to run post-build automatically

https://bugzilla.tianocore.org/show_bug.cgi?id=220

Currently, the post-build scripts PostBuild.bat/PostBuild.sh in DuetPkg
need to be run manually. Especially for Windows batch script, it also
requires users to set the build options (like tool chain, target and arch)
in file Conf/target.txt. If users using command line options via '-t' or
'-a', the post-build script won't work properly.

The package DSC files now support the feature to execute post-build script
automatically by adding a 'POSTBUILD' definition. This feature also passes
the build options into the post-build script as parameters. This commit
uses this feature to make the post-build works for DuetPkg more
user-friendly. Also, ReadMe.txt is updated to reflect the new steps for
UEFI Emulation (DUET) development.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>