etcupdate: Add a -m flag to change the make binary that's run

This will allow release/Makefile to forward on ${MAKE} to allow building
on non-FreeBSD systems where ${MAKE} is something other than make, as
make is typically GNU make in such situations.

Reviewed by: jhb, pauamma
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D35854