magenta: boot: Add a function prototype for retrieving the framebuffer.

This will hopefully go away when magenta's dependence on the firmware to set
up the framebuffer goes away. In the mean time, configurations that need to
boot magenta should implement this callback somewhere, likely in their board
driver.

Change-Id: Ic9cad24b73bff19722cba713fd59653e144a09dc
1 file changed