I am having the exact same problem with my quad wandboard, only I'm using Android.
However, I was able to confirm that my system is booting. Input and output are by default routed to the serial port (115.2 kbaud, 8 bits, 1 stop bit, no parity).
If yocto is configured the same way, all you need is a null modem cable.
I probed the signal pins on the HDMI connector with a scope and I found that both clock and data are there. I am measuring a clock frequency of about 76 MHz with a 0.5 V swing (single ended), which seems about right.
Still, my screen stays dark. My monitor is an older DELL with a DVI port. I have tried another monitor with the same result, so it seems that it's probably not the monitor. It could be the cable (a really cheap one), but I don't have a good way to test it.
Have you made any progress meanwhile?