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"
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