Vlv2TbltDevicePkg: Allow BaseTools to run from sources

Vlv2TbltDevicePkg: Allow BaseTools to run from sources

https://bugzilla.tianocore.org/show_bug.cgi?id=258

Update bld_vlv.bat to run BaseTools build command with a call
statement to support running the build command from python
sources.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: David Wei <david.wei@intel.com>
Cc: Mang Guo <mang.guo@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>