Implement XMODEM bootloader

[troeger]

The bootstrap loader should wait on the serial line for receiving a Python script to be executed.

My recommendation is the XMODEM protocol - the sending clients are given for all operating systems and the protocol is trivial enough to go directly into the bootloader.

Example implementations for the server / receiver side are here:

https://github.com/Bandicoot/xmodem.c/blob/master/xmodem.c (without CRC, as described below)
https://github.com/crazyleen/rx/blob/master/xmodem.c (with CRC)

The protocol specification is here:

http://pauillac.inria.fr/~doligez/zmodem/ymodem.txt (page 20ff)

The server / receiver protocol (XMODEM without CRC) works like this:

• <soh> is 0x01
• <eot> is 0x04
• <ack> is 0x06
• <nak> is 0x15
• <can> is 0x18
• Serial line with 8N1 setup, 8bit characters sent
• Receive protocol blocks of the following layout:
  • <soh>
  • <block number> (starts at 0x01, increments by 1, and wraps 0x0FF to 0x00)
  • <255-block number> (ones complement of the block number)
  • <--128 data bytes-->
  • <cksum> (sum of all data bytes, overflow ignored)
• While waiting for data on the serial line, perform a timeout after 10s, and then send <nak>.
• On protocol start, the sender will simply wait for the first <nak>. The server therefore may skip the initial timeout.
• During the receiving of a block, the timeout should be 1 second, and then <nak> is sent.
• Receiving of a correct protocol block is acknowledged by sending <ack>.
• Receiving of an incorrect protocol block is acknowledged by sending <nak>.
• Sender sends an <eot> if no more data is available, must be acknowledged by sending <ack>.
• The last block is filled up to also have 128 data abytes.
• Same block number being sent is ok and should be ignored.
• In case anything goes horribly wrong, kill the connection by sending <can>.