board/minnowboard-max: Add a X-based graphical defconfig

board/minnowboard-max: Add a X-based graphical defconfig

This commit adds a new defconfig to build a X-based
graphical system for the Minnowboard MAX board.

  * The 'openbox' windows manager is chosen because it's simple
    and lightweigth.

  * Basic X apps are enabled (such as xrandr, xterm), so we
    can at least get a console and change video mode.

  * ALSA default configuration is provided, so HDMI audio
    works out-of-the-box.

  * OpenGL is supported.

Tested on Minnoboard Turot (which is Minnowboard Max compatible).

[Peter: drop unneeded/specific toolchain config options]
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>