Platform/Intel/KabylakeOpenBoardPkg: Fix build.

The build for KabylakeOpenBoardPkg is presently broken due to
WORKSPACE_PLATFORM_BIN being defined to a marco of the same name in
KabylakeOpenBoardPkg/KabylakeRvp3/build_config.cfg

Setting this macro to an empty string resolves the issue.

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Michael Kubacki <michael.a.kubacki@intel.com>
Signed-off-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Michael Kubacki <michael.a.kubacki@intel.com>