Update split serial code to use driver pattern (#7990)

* Move avr serial code to drivers

* Update src+= serial.c to driver pattern

common_features.mk

@@ -469,9 +469,15 @@ ifeq ($(strip $(SPLIT_KEYBOARD)), yes)
         QUANTUM_SRC += $(QUANTUM_DIR)/split_common/transport.c
         # Functions added via QUANTUM_LIB_SRC are only included in the final binary if they're called.
         # Unused functions are pruned away, which is why we can add multiple drivers here without bloat.
-        QUANTUM_LIB_SRC += $(QUANTUM_DIR)/split_common/serial.c \
-                           i2c_master.c \
+        QUANTUM_LIB_SRC += i2c_master.c \
                            i2c_slave.c
+
+        SERIAL_DRIVER ?= bitbang
+        ifeq ($(strip $(SERIAL_DRIVER)), bitbang)
+            QUANTUM_LIB_SRC += serial.c
+        else
+            QUANTUM_LIB_SRC += serial_$(strip $(SERIAL_DRIVER)).c
+        endif
     endif
     COMMON_VPATH += $(QUANTUM_PATH)/split_common
 endif