Allow ESP32 to be flashed using Hackflight

[PepMS]

Feature / Bug description

Using a request MSP message, Hackflight has to boot ESP32 into flash mode so it can receive the firmware via the USB Serial of the BonaDrone's FC.
Simple sketch showing the procedure is provided in the following link.

Additional information on how to do so can be found at: #64

Expected behavior

With the Hackflight firmware loaded into the STM32L4, an MSP message request is sent via Serial. Then, the MSP is boot into flash mode. Once this message is sent and received properly, one should be able to flash the ESP32 either using the Arduino IDE or via the terminal using the provided python scropt.