esp32s2: 'make flash': Allow customizing the esptool flags

This can be useful so that e.g., on a Kaluga when programming via
the FTDI chip, you can override the variable to specify "--after=hard_reset"
to automatically return to running CircuitPython, choose a different
baud rate (921600 is about 2s faster than 460800), etc:
    make BOARD=espressif_kaluga_1 ESPTOOL_FLAGS="-b 921600 --before=default_reset --after=hard_reset"

Author: jepler

ports/esp32s2/Makefile

@@ -300,6 +300,8 @@ ESP_AUTOGEN_LD = $(BUILD)/esp-idf/esp-idf/esp32s2/esp32s2_out.ld $(BUILD)/esp-id
 
 FLASH_FLAGS = --flash_mode $(CIRCUITPY_ESP_FLASH_MODE) --flash_freq $(CIRCUITPY_ESP_FLASH_FREQ) --flash_size $(CIRCUITPY_ESP_FLASH_SIZE)
 
+ESPTOOL_FLAGS ?= -b 460800 --before=default_reset --after=no_reset write_flash
+
 all: $(BUILD)/firmware.bin $(BUILD)/firmware.uf2
 
 .PHONY: esp-idf-stamp
@@ -338,10 +340,10 @@ $(BUILD)/firmware.uf2: $(BUILD)/circuitpython-firmware.bin
 	$(Q)$(PYTHON3) $(TOP)/tools/uf2/utils/uf2conv.py -f 0xbfdd4eee -b 0x0000 -c -o $@ $^
 
 flash: $(BUILD)/firmware.bin
-	esptool.py --chip esp32s2 -p $(PORT) -b 460800 --before=default_reset --after=no_reset write_flash $(FLASH_FLAGS) 0x0000 $^
+	esptool.py --chip esp32s2 -p $(PORT) $(ESPTOOL_FLAGS) write_flash $(FLASH_FLAGS) 0x0000 $^
 
 flash-circuitpython-only: $(BUILD)/circuitpython-firmware.bin
-	esptool.py --chip esp32s2 -p $(PORT) -b 460800 --before=default_reset --after=no_reset write_flash $(FLASH_FLAGS) 0x10000 $^
+	esptool.py --chip esp32s2 -p $(PORT) $(ESPTOOL_FLAGS) write_flash $(FLASH_FLAGS) 0x10000 $^
 
 include $(TOP)/py/mkrules.mk