Adding a MPI helper

[thierry]

Summary

This adds a file ports/Mk/Uses/mpi.mk.
Its aim is to simplify the use of MPI in the ports Makefile.
It makes it easier to provide an option for the user to choose between MPICH and OpenMPI when applicable.

Note: mpich2 and openmpi3 are not handled, because they will be deprecated.

Test Plan

• Take some port using MPI (they are mostly in the category science)
• Add USES+=mpi
• Remove the MPI related dependency lines
• Adapt CONFIGURE_ARGS / CMAKE_ARGS / whatever with the provided definitions
• make configure if available and check carefully its result
• make build.