clearing the screen with Open Firmware

[Hollis Blanchard]

I realized that grub_ofconsole_cls() wasn't working at all on my G3 
with video console. With a little bit of experimentation on that and 
serial console on the briQ, I've determined that using *both* the ANSI 
escape sequence and a literal ^L seems to work. Please let me know if 
you see odd video artifacts after this (that includes Sparc, since 
Sparc shares this driver).

I've also cleared the screen after resuming GRUB from the "suspend" 
command, since GRUB has no idea what OF might have output, and so no 
idea what the current curson position should be.

-Hollis