Forward button presses to FPGA in FPGA download mode

[renzenicolai]

When the FPGA bitstream has been downloaded and the ESP32 is waiting for a new start command from the PC to load a new bitstream the ESP32 should forward button presses to the FPGA as simple SPI transfers.

The button presses are sent to a queue (that can be added to the FPGA downloader function as an argument from main, have a look at the other functions of the firmware that use buttons) and the ice40 driver has a function to send data to the ICE40 via SPI: esp_err_t ice40_send(ICE40* device, const uint8_t* data, uint32_t length);

The FPGA people specified the following:

Packet would be f4 xx xx yy yy with the xx xx being current state and yy yy the change mask.
(the F4 byte at the start is a command byte / constant)

[FransFaase]

The above commits seem to implement the required function. I tested this with Buttons.bin.

Bits are mapped to the following keys:
0 - joystick down
1 - joystick up
2 - joystick left
3 - joystick right
4 - joystick press
5 - home
6 - menu
7 - select
8 - start
9 - accept
10 - back

[FransFaase]

Did some code clean-up. Button processing has now be moved to fpga_process_events. I decided on this generic name, because maybe also other events will be send to the FPGA in the future using this function.

The commit also includes a version of the fpga.py which keeps the port open and echos messages from USB serial port about the progress of the download or possible error messages. This could aid debugging.