contrib: Add caching of build

This is a primitive way of caching builds of contrib.  Basically, a hash entry
is created from all the *.mak files in contrib.  This include all rules.mak of
individual packages and config.mak generated by the bootstrap script.

When caching is enabled, the installation is done in the cache and a symbolic
link is created to it.

Caching can be enabled with `--cache-dir=/tmp/jami-contrib-cache --cache-builds'
or any other path for cache-dir.  Not providing the `cache-builds` option will
bypass the cache.

Change-Id: I58aa0295664113792518851ede1c568495a33e59