Add wayland-server support to nullws

Adds support for running a wayland compositor off of nullws.
This allows for platforms that have trouble bringing up fbdev and
hwcomposer to run a wayland compositor using null EGL platform for the
compositor.

tested using nexus5, qml-compositor and hellogl_es2 client.
Merge pull request #233 from heeen/null-compositor

Add wayland-server support to nullws