Add support for soft serial to ATmega32U2 (#10204)

* Add support for soft serial to ATmega32U2

* Update drivers/avr/serial.c

* Update drivers/avr/serial.c

* Update drivers/avr/serial.c

* Fix comment

* Update drivers/avr/serial.c

common_features.mk

@@ -443,8 +443,10 @@ ifeq ($(strip $(SPLIT_KEYBOARD)), yes)
         # Functions added via QUANTUM_LIB_SRC are only included in the final binary if they're called.
         # Unused functions are pruned away, which is why we can add multiple drivers here without bloat.
         ifeq ($(PLATFORM),AVR)
-            QUANTUM_LIB_SRC += i2c_master.c \
-                               i2c_slave.c
+            ifneq ($(NO_I2C),yes)
+                QUANTUM_LIB_SRC += i2c_master.c \
+                                   i2c_slave.c
+            endif
         endif
 
         SERIAL_DRIVER ?= bitbang