scripts: west_commands: runners: esp32: add flash encryption option

Add an argument `--esp-encrypt` to flash an ESP32 series chip with
encrypted flash.

The content of the binary is encrypted on the fly by the chip. It is
not required to know the encryption key. However, the fuses that
disable encrypted flash download must be kept enabled for this to work.

Signed-off-by: Martin Jäger <martin@libre.solar>