uefi: Implement the new magenta_framebuffer instead of board_framebuffer.

The framebuffer should generally be abstracted behind the "display" object and
not exposed directly as part of the public board API. This change makes the
UEFI board use the magenta_framebuffer callback instead, who's purpose is more
explicit, which is separated from the board_* functions, and which will
hopefully go away at some point in the future.

Change-Id: I68fed541448096019b99712d901ea4a01e32e411
1 file changed