add 'mavrdude' target into tmk_core/avr.mk (#3986)

* add 'mavrdude' target into tmk_core/avr.mk

I made it a little convenient when writing the same binary to multiple Pro Micro.

* rename target name 'mavrdude' to 'avrdude-loop'

* modify docs/flashing.md about avrdude-loop

* mdify docs/flashing.md again

* modifi docs/flashing.md 3rd

2 files changed, 17 insertions, 1 deletions

diff --git a/docs/flashing.md b/docs/flashing.md
index e5fe59f8f..66c08a361 100644
--- a/docs/flashing.md
+++ b/docs/flashing.md
@@ -78,6 +78,12 @@ or
 
     make <keyboard>:<keymap>:avrdude
 
+or if you want to flash multiple boards, use the following command
+
+    make <keyboard>:<keymap>:avrdude-loop
+
+When you're done flashing boards, you'll need to hit Ctrl + C or whatever the correct keystroke is for your operating system to break the loop.
+
 ## Halfkay
 
 Halfkay is a super-slim protocol developed by PJRC that uses HID, and come on all Teensys (namely the 2.0).
diff --git a/tmk_core/avr.mk b/tmk_core/avr.mk
index 2f955b18f..add904c9c 100644
--- a/tmk_core/avr.mk
+++ b/tmk_core/avr.mk
@@ -169,7 +169,8 @@ dfu-ee: $(BUILD_DIR)/$(TARGET).hex $(BUILD_DIR)/$(TARGET).eep
         fi
         $(DFU_PROGRAMMER) $(MCU) reset
 
-avrdude: $(BUILD_DIR)/$(TARGET).hex check-size cpfirmware
+define EXEC_AVRDUDE
+        USB= ;\
         if $(GREP) -q -s Microsoft /proc/version; then \
                 echo 'ERROR: AVR flashing cannot be automated within the Windows Subsystem for Linux (WSL) currently. Instead, take the .hex file generated and flash it using AVRDUDE, AVRDUDESS, or XLoader.'; \
         else \
@@ -191,6 +192,15 @@ avrdude: $(BUILD_DIR)/$(TARGET).hex check-size cpfirmware
                 sleep 1; \
                 avrdude -p $(MCU) -c avr109 -P $$USB -U flash:w:$(BUILD_DIR)/$(TARGET).hex; \
         fi
+endef
+
+avrdude: $(BUILD_DIR)/$(TARGET).hex check-size cpfirmware
+        $(call EXEC_AVRDUDE)
+
+avrdude-loop: $(BUILD_DIR)/$(TARGET).hex check-size cpfirmware
+        while true; do \
+            $(call EXEC_AVRDUDE) ; \
+        done
 
 # Convert hex to bin.
 bin: $(BUILD_DIR)/$(TARGET).hex