The plugin is intentionally built using a custom Makefile,
because it's used by gbuild, so I don't want to build the plugin
using gbuild too. It is also intentionally not placed under workdir/,
as that is cleaned by 'make clean', the plugin is cleaned only
by 'make distclean', so that cleaning it doesn't cause ccache misses.
No actual functionality in the plugin itself yet.

Change-Id: Ic05eba8d6260eec123c9e699eb5385abfe1b832f