BaseTools/build: Expand PREBUILD/POSTBUILD DSC actions

BaseTools/build: Expand PREBUILD/POSTBUILD DSC actions

https://bugzilla.tianocore.org/show_bug.cgi?id=670

* Extend PREBUILD/POSTBUILD define values to support more than
  one argument.
* Delay normalization of PREBUILD/POSTBUILD define values
  until all arguments in the define values can be processed.
* Convert PREBUILD/POSTBUILD build define value arguments
  that are WORKSPACE or PACKAGES_PATH relative paths to
  absolute paths.
* Append -p PlatformFile, --conf=ConfDirectory, and build target
  flags to command line used to execute PREBUILD/POSTBUILD
  actions.
* Remove PrebuildScript and PostbuildScript fields from the
  Build class and use Prebuild and Postbuild fields instead.

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