weston: add weston-imx variant when using imx-gpu-viv

weston: add weston-imx variant when using imx-gpu-viv

This variant contains various optimizations for i.MX processors.

For instance, on i.MX6/7 devices with GPU, the gl-renderer needs to be
enabled for the fbdev-backend which was removed from upstream weston
long time ago.

Also, weston-imx adds support for G2D which is enabled by default, this
patch makes sure to disable it when imx-gpu-g2d isn't selected.

The tag version rel_imx_4.9.51_8mq_ga proved to work fine on both
i.MX6Q/DL and i.MX8MQ processors.

Here are the commands used to start weston on i.MX6Q:
- Using 3D GPU (gl-renderer):
 # weston --tty=1 --device=/dev/fb0
- Using 2D GPU (G2D):
 # weston --tty=1 --device=/dev/fb0 --use-g2d=1

Upstream repository:
https://source.codeaurora.org/external/imx/weston-imx/

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Arnout: add comment why no --enable option is passed]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>