Expose mono_build_root to mcs build system

Expose mono_build_root to mcs build system

In an out-of-tree build the Makefile which contains get-monolite-latest
doesn't exist in the source directory (which $(topdir)/.. would point to)
but in the build directory.

In order to invoke get-monolite-latest we need to actually call the Makefile
in the correct directory which requires exposing the build directory to the
mcs build system.