Add --fbdev option to psplash like --rotation.

A new fbdev file is needed and should be overrided if needed.
The number contained in fbdev will be used as X in /dev/fbX.

By overriding fbdev you can have a splash screen on another
framebuffer than /dev/fbO.

The getenv(FBDEV) line has been replaced by this option since
the environment variables are not read yet when psplash starts.

Signed-off-by: Julien Gueytat <contact@jgueytat.fr>
Signed-off-by: Ross Burton <ross.burton@intel.com>