GD firmware upload and flash

[bs-github]

This PR adds the capability to

• upload a GD firmware to the LittleFS on the ESP32 and to
• flash the GD firmware either from LittleFS or the embedded one if compiled in

To archive this, the following has been added / changed

• add modified copy of the frontend firmware_update module as gd_firmware_update / GD Firmware Update
• add precheck to recognize GD firmwares and allow to upload only valid ones
• change the flash function to fit into the normal loop design
• trigger a GD firmware flash automatically after upload
• trigger a GD firmware flash on demand (web UI button in EVSE section)
• make the flash function more roboust - even recover the GD after ESP32 reboot
• add human readable 10% steps flash progress logging
• enable the Debug module (disable for release!)

[bs-github]

Would be nice to add the intel hex to bin conversion for an uploaded firmware.