Move avrdude flashing script from individual keyboard rules.mk to common location and improve script.

Port detection script is now more informative and better handles first time flashing of a Pro Micro

keyboards/lets_split/rules.mk

@@ -74,15 +74,3 @@ USE_I2C = yes
 SLEEP_LED_ENABLE = no    # Breathing sleep LED during USB suspend
 
 CUSTOM_MATRIX = yes
-
-avrdude: build
-	ls /dev/tty* > /tmp/1; \
-	echo "Reset your Pro Micro now"; \
-	while [ -z $$USB ]; do \
-	  sleep 1; \
-	  ls /dev/tty* > /tmp/2; \
-	  USB=`diff /tmp/1 /tmp/2 | grep -o '/dev/tty.*'`; \
-	done; \
-	avrdude -p $(MCU) -c avr109 -P $$USB -U flash:w:$(BUILD_DIR)/$(TARGET).hex
-
-.PHONY: avrdude