xilinx_async_fifo: wrapper around native fifo block
hdmi_core: expose vblank indicator
axi_dma_reader: fetch data from axi in 16 word bursts
zybo_hdmi_axi: implement a framebuffer at 0x10000000

- control registers:
  - 0x40000000: write any value to reset fifo
  - 0x40000004: write 1 to enable framebuffer scanout
  - 0x40000008: write RGB:24 value to display on fifo underrun