Add definition based on currently-selected serial driver. (#10716)

* Add definition based on currently-selected serial driver.

* Apply suggestions from code review

Suggestions from zvecr.

Co-authored-by: Joel Challis <git@zvecr.com>

Co-authored-by: Joel Challis <git@zvecr.com>

common_features.mk

@@ -450,6 +450,7 @@ ifeq ($(strip $(SPLIT_KEYBOARD)), yes)
         endif
 
         SERIAL_DRIVER ?= bitbang
+        OPT_DEFS += -DSERIAL_DRIVER_$(strip $(shell echo $(SERIAL_DRIVER) | tr '[:lower:]' '[:upper:]'))
         ifeq ($(strip $(SERIAL_DRIVER)), bitbang)
             QUANTUM_LIB_SRC += serial.c
         else