serial: Support custom baud rates on linux

The function to do this is contained in custom_baud.c because
of broken include stuff.

Change-Id: I2a20f9182cb85e7bce5d6654a2caf20e6202b195
Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Reviewed-on: https://review.coreboot.org/20224
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Makefile

@@ -921,7 +921,7 @@ NEED_LIBUSB1 += CONFIG_CH341A_SPI
 endif
 
 ifneq ($(NEED_SERIAL), )
-LIB_OBJS += serial.o
+LIB_OBJS += serial.o custom_baud.o
 endif
 
 ifneq ($(NEED_POSIX_SOCKETS), )