Enable CLI flashing via mdloader (#14729)

* Enable CLI flashing via mdloader

* remove flag

* Update qmk flash help

platforms/arm_atsam/flash.mk

@@ -3,9 +3,20 @@
 # Architecture or project specific options
 #
 
+MDLOADER_CLI ?= mdloader
+
+define EXEC_MDLOADER
+	$(MDLOADER_CLI) --first --download $(BUILD_DIR)/$(TARGET).bin --restart
+endef
+
+mdloader: bin
+	$(call EXEC_MDLOADER)
+
 flash: bin
 ifneq ($(strip $(PROGRAM_CMD)),)
 	$(UNSYNC_OUTPUT_CMD) && $(PROGRAM_CMD)
+else ifeq ($(strip $(ARM_ATSAM)),SAMD51J18A)
+	$(UNSYNC_OUTPUT_CMD) && $(call EXEC_MDLOADER)
 else
 	$(PRINT_OK); $(SILENT) || printf "$(MSG_FLASH_ARCH)"
 endif