With help from support, I have enabled HDMI on my board, and just for reference here is the steps I took:
First I enabled it in arch/arm/boot/dts/armstonea9r2q.dts:
#define CONFIG_ARMSTONEA9R2_MXCFB0 DISPLAY_LVDS0#define CONFIG_ARMSTONEA9R2_MXCFB1 DISPLAY_HDMI
I edited buildroot-2016.05-fsimx6-V3.0/board/f+s/common/etc/X11/xorg.conf to point to /dev/fb0
After booting, I run:
echo 0 > /sys/class/graphics/fb0/blankecho 0 > /sys/class/graphics/fb1/blank
To ensure none of the outputs are blank, I now see my desktop on HDMI output. Hooray! 
Trying to get support for hardware accelerated video decoding, I ran from the manual:
gst-launch-1.0 filesrc location=video.avi typefind=true ! aiurdemux ! vpudev ! imxv4l2sink
Please note there is a confirmed typo in this (correction on the way), the correct command should be:
gst-launch-1.0 filesrc location=video.avi typefind=true ! aiurdemux ! vpudec ! imxv4l2sink
But when running this I get:
- # gst-launch-1.0 filesrc location=gravity.mp4 typefind=true ! aiurdemux ! vpudec ! imxv4l2sink
- (gst-plugin-scanner:585): GStreamer-CRITICAL **:
- gst_element_class_add_pad_template: assertion
- 'GST_IS_PAD_TEMPLATE (templ)' failed
- Can't get caps from capture device, use the default setting.
- Perhaps haven't capture device.
- display(/dev/fb0) resolution is (1920x1080).
- [INFO] Product Info: i.MX6Q/D/S
- [INFO] Product Info: i.MX6Q/D/S
- ====== IMXV4L2SINK: 4.0.9 build on Feb 15 2017 15:53:16. ======
- Setting pipeline to PAUSED ...
- ERROR: Pipeline doesn't want to pause.
- Setting pipeline to NULL ...
- Freeing pipeline ...
Do we lack VPU hardware support currently?
