serial: Add Darwin/macOS support for custom and >230400 baudrates

This change is based on the patch proposed by Denis Ahrens in
https://review.coreboot.org/c/flashrom/+/67822

Change-Id: I3e6b88d2b4c2a130b16456752681fd9f807bf6f0
Signed-off-by: Peter Stuge <peter@stuge.se>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/70571
Original-Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/73480
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/Makefile b/Makefile
index 95aeae6..dde09a5 100644
--- a/Makefile
+++ b/Makefile
@@ -796,9 +796,13 @@
 ifeq ($(TARGET_OS), Linux)
 LIB_OBJS += custom_baud_linux.o
 else
+ifeq ($(TARGET_OS), Darwin)
+LIB_OBJS += custom_baud_darwin.o
+else
 LIB_OBJS += custom_baud.o
 endif
 endif
+endif
 
 USE_SOCKETS := $(if $(call filter_deps,$(DEPENDS_ON_SOCKETS)),yes,no)
 ifeq ($(USE_SOCKETS), yes)