Silence the functions of shim protocol

When grub2 invokes the functions of shim protocol in gfx mode,
OutputString in shim could distort the screen.

Signed-off-by: Gary Ching-Pang Lin <glin@suse.com>

Conflicts:
	shim.c

(modified by pjones to include some newer Prints that weren't there when
Gary did the initial work here.)