drivers/pc80/rtc: Rename mc146818rtc_early.c -> _romcc.c

And don't link it. It's for ROMCC.

To make code happy that uses the ROMCC interface read_option(),
read_option_lowlevel() is ported to mc146818rtc.c along with
a message to use get_option() instead.

Change-Id: I54ea08de034766c8140b320075d36d5e811582fa
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/19663
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Philippe Mathieu-Daudé <philippe.mathieu.daude@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/drivers/uart/util.c

@@ -24,7 +24,7 @@ unsigned int default_baudrate(void)
 	static const unsigned baud[8] =
 		{ 115200, 57600, 38400, 19200, 9600, 4800, 2400, 1200 };
 	unsigned b_index = 0;
-#if defined(__PRE_RAM__)
+#if defined(__ROMCC__)
 	b_index = read_option(baud_rate, 0xff);
 #else
 	if (get_option(&b_index, "baud_rate") != CB_SUCCESS)