Fix release build of mscorlib from build.sh

Fix release build of mscorlib from build.sh

The mscorlib build in build.sh was not correctly flowing __BuildType
into MSBuild, which caused us to always build a Debug mscorlib.

While I was in the area, I removed the defines for OSGroup (which are
unused, as we use __BuildOS instead) and cleaned up some of the property
switches so we always pass prefixed properties (the bare names are the
computed ones from our targets in msbuild).