Disable the eglfs input handlers by Environment variable

Added a Enironment variable to make it possible to disable the
automatic installation of EVDEV input handlers.

This is needed if you want to use your own generic plugin instead,
which also uses evdev

Change-Id: I17d47008c10999bf918db62a22a3b6a38d7abb80
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>