Design & implement new upload protocol

[yaqwsx]

The current upload protocol suffers from missing flow control on the UART line. We cannot establish a hardware flow control as there is none on the existing boards. Therefore, occasionally it corrupts the files - parts are missing, and leading/slash characters are altered as the bytes are lost during transmission. This is currently nastily solved by delays in the uploader tool.

We should implement flow control into the protocol - it could possibly map directly to the implementation as it is now (start file, add chunk, commit file).

Also, we should add support for CRC checking - just to be sure and detect errors.