unified build scheme: add and document the "unified" driving engine

common.tmpl will be used together with the template build file, and is
the engine that connects the information gathered from all the
build.info files with making the build file itself.

This file expects there to be a template section in the build file
template that defines a number perl functions designed to return
strings with appropriate lines for the build system at hand.  The
exact functions, what they can expect as arguments and what output
they're expected to produce is documented in Configurations/README.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>