custom_baud: move Linux specific code into own file

Handle system specific code in an own file like i2c_helper_linux.c.
The build system decides when to build it.

Change-Id: I0744e769dcc6000483e7256105903a87e927ee77
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.de>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/57990
Original-Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/71425
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/Makefile b/Makefile
index 8aac967..9a5abd9 100644
--- a/Makefile
+++ b/Makefile
@@ -721,7 +721,12 @@
 endif
 
 ifneq ($(NEED_SERIAL), )
-LIB_OBJS += serial.o custom_baud.o
+LIB_OBJS += serial.o
+ifeq ($(TARGET_OS), Linux)
+LIB_OBJS += custom_baud_linux.o
+else
+LIB_OBJS += custom_baud.o
+endif
 endif
 
 ifneq ($(NEED_POSIX_SOCKETS), )