Version 2.4.1

Bug fixes:

* --port argument not correctly parsed in v2.4.0
* Corrected pyserial dependency to require pyserial v3.0 or newer

Version 2.4.0:

* If no serial port is specified, try all serial ports until an Espressif
  device responds.
* Print device MAC on every command, after connecting.
* New option `--before no_reset_no_sync` for use when immediately reusing the
  serial port with an already-running loader.
* Fix spurious load_ram error when executed code immediately resets the UART
  baud rate..
* Produce an error if load_ram is going to clobber bootloader stub memory (can use
  --no-stub to work around this).
* Display a helpful error if the incompatible 'serial' module was installed
  instead of 'pyserial'.
* Explicitly close the serial port at end of main() function (allows calling
  esptool.main() from another Python program for basic scripting.)
* Workaround Windows usbser.sys (USB CDC-ACM device) driver bug setting DTR & RTS
* Flasher stub build updates to work with recent ESP-IDF & ESP8266 SDK headers
* Flasher stub now sends correct length field in protocol responses (esptool.py
  was ignoring this field, but some other tools were not).

* Move the loader stub to the very end of IRAM (useful when using load_ram command).
* `elf2image --version=2` is now compatible with recent ESP8266 SDK bootloader
  and produces the same binary output as ESP8266 SDK gen_app.py.
* Remove unsupported flash size 4MB-c2

* Print chip frequency rating values in Features list, for chips where these are
  set.
* Working load_ram command.
* espefuse.py: Support setting the baud rate.
* New --override-vddsdio option allows setting the VDDSDIO regulator to 1.8V,
  1.9V or "OFF" when connecting (to override the default value read from efuse).
  To avoid hardware damage, forcing VDDSDIO to 3.3V is not supported.

Thanks to @devanlai, @andrewwakeling, @Callahan93, @tim-nordell-nimbelink, @cefn
and @nkolban for contributing to this release.