Add st-flash flash target (qmk#9964)

* Add `st-flash` flash target

Add support for flashing the firmware via the `st-flash` utility from
the STLink Tools package (https://github.com/stlink-org/stlink).

* Add `st-flash` to the `qmk flash -b` output

tmk_core/chibios.mk

@@ -318,12 +318,14 @@ ifneq ("$(SERIAL)","")
 endif
 
 ST_LINK_ARGS ?=
+ST_FLASH_ARGS ?=
 
 # List any extra directories to look for libraries here.
 EXTRALIBDIRS = $(RULESPATH)/ld
 
 DFU_UTIL ?= dfu-util
 ST_LINK_CLI ?= st-link_cli
+ST_FLASH ?= st-flash
 
 define EXEC_DFU_UTIL
 	until $(DFU_UTIL) -l | grep -q "Found DFU"; do\
@@ -357,6 +359,9 @@ dfu-util-split-right: dfu-util
 st-link-cli: $(BUILD_DIR)/$(TARGET).hex sizeafter
 	$(ST_LINK_CLI) $(ST_LINK_ARGS) -q -c SWD -p $(BUILD_DIR)/$(TARGET).hex -Rst
 
+st-flash: $(BUILD_DIR)/$(TARGET).hex sizeafter
+	$(ST_FLASH) $(ST_FLASH_ARGS) --reset --format ihex write $(BUILD_DIR)/$(TARGET).hex
+
 
 # Autodetect teensy loader
 ifndef TEENSY_LOADER_CLI