Remove DirectFB support for i.MX6

Remove DirectFB support for i.MX6

The DirectFB project is dead upstream for a while and the backend is
deprecated so it is better to drop it and avoid new users to depend on
it and instead to focus in the current technologies for new projects.

This commit removes everything related to the DirectFB support so if
someone wish to revert it locally it can be easily done.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Daiane Angolini <daiane.angolini@freescale.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>