BaseTools: Update Build tool to support multiple workspaces

BaseTools: Update Build tool to support multiple workspaces

WORKSPACE is still kept.
New PACKAGES_PATH is introduced to specify the additional WORKSPACEs.
In PACKAGES_PATH, ';' is separator in Windows, ':' is separator in Linux.

Build directory is in WORKSPACE. Package, BaseTools and Conf directory
will be found from WORKSPACE and PACKAGES_PATH.

In implementation, BaseTools adds MultipleWorkspace class for
the file path conversion from WORKSPACE and PACKAGES_PATH.

Verify two tree layouts.
Root\edk2\MdePkg
Root\edk2\MdeMdeModulePkg
Root\edk2\...
1. set WORKSPACE=Root\edk2
2. set WORKSPACE=Root, and set PACKAGES_PATH=Root\edk2

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Li YangX <yangx.li@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18579 6f19259b-4bc3-4df7-8a09-765794883524