Some BIOSes return 0xe0 for extended characters. Just assume that

anything over 0x80 is an extended character.

src/arch/i386/firmware/pcbios/bios_console.c

 	character = ( keypress & 0xff );
 
 	/* If it's a normal character, just return it */
-	if ( character )
+	if ( character < 0x80 )
 		return character;
 
 	/* Otherwise, check for a special key that we know about */