Add avrdude split EE_HANDS flash commands (#5414)

* Add EEPROM avrdude flashing commands for setting handedness

* Update docs with info on flashing EEPROM handedness files with commands

docs/config_options.md

@@ -208,9 +208,13 @@ There are a few different ways to set handedness for split keyboards (listed in
 
 1. Set `SPLIT_HAND_PIN`: Reads a pin to determine handedness. If pin is high, it's the left side, if low, the half is determined to be the right side
 2. Set `EE_HANDS` and flash `eeprom-lefthand.eep`/`eeprom-righthand.eep` to each half
+   * For boards with DFU bootloader you can use `:dfu-split-left`/`:dfu-split-right` to flash these EEPROM files
+   * For boards with Caterina bootloader (like stock Pro Micros), use `:avrdude-split-left`/`:avrdude-split-right`
 3. Set `MASTER_RIGHT`: Half that is plugged into the USB port is determined to be the master and right half (inverse of the default)
 4. Default: The side that is plugged into the USB port is the master half and is assumed to be the left half. The slave side is the right half
 
+#### Defines for handedness
+
 * `#define SPLIT_HAND_PIN B7`
   * For using high/low pin to determine handedness, low = right hand, high = left hand. Replace `B7` with the pin you are using. This is optional, and if you leave `SPLIT_HAND_PIN` undefined, then you can still use the EE_HANDS method or MASTER_LEFT / MASTER_RIGHT defines like the stock Let's Split uses.
 

tmk_core/avr.mk

@@ -223,7 +223,11 @@ define EXEC_AVRDUDE
 			printf "Waiting for $$USB to become writable."; \
 			while [ ! -w "$$USB" ]; do sleep 0.5; printf "."; done; echo ""; \
 		fi; \
+		if [ -z "$(1)" ]; then \
 			avrdude -p $(MCU) -c avr109 -P $$USB -U flash:w:$(BUILD_DIR)/$(TARGET).hex; \
+		else \
+			avrdude -p $(MCU) -c avr109 -P $$USB -U flash:w:$(BUILD_DIR)/$(TARGET).hex -U eeprom:w:$(QUANTUM_PATH)/split_common/$(1); \
+		fi \
 	fi
 endef
 
@@ -235,6 +239,12 @@ avrdude-loop: $(BUILD_DIR)/$(TARGET).hex check-size cpfirmware
 		$(call EXEC_AVRDUDE) ; \
 	done
 
+avrdude-split-left: $(BUILD_DIR)/$(TARGET).hex check-size cpfirmware
+	$(call EXEC_AVRDUDE,eeprom-lefthand.eep)
+
+avrdude-split-right: $(BUILD_DIR)/$(TARGET).hex check-size cpfirmware
+	$(call EXEC_AVRDUDE,eeprom-righthand.eep)
+
 # Convert hex to bin.
 bin: $(BUILD_DIR)/$(TARGET).hex
 	$(OBJCOPY) -Iihex -Obinary $(BUILD_DIR)/$(TARGET).hex $(BUILD_DIR)/$(TARGET).bin