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/meson.build b/meson.build
index 9d35cc4..4620860 100644
--- a/meson.build
+++ b/meson.build
@@ -294,8 +294,12 @@
 
 # raw serial IO
 if need_serial
-  srcs += 'custom_baud.c'
   srcs += 'serial.c'
+  if host_machine.system() == 'linux'
+    srcs += 'custom_baud_linux.c'
+  else
+    srcs += 'custom_baud.c'
+  endif
 endif
 
 prefix = get_option('prefix')