Firmware update example

[dakejahl]

Added an example script to update the firmware on a CAN node. I used the gui_tool project as an example to follow.

I don't like this

node.defer(4, request_update)

but it does work -- from what I understand sending the BeginFirmwareUpdate request triggers the node to reboot. We then send BeginFirmwareUpdate again such that the bootloader catches it and actually starts updating. Do I understand this correctly?

[dakejahl]

@julianoes Thanks for the review. I made the suggested changes and shortened the argument names.

[tridge]

@dakejahl I've pushed a few fixes to this branch. Key changes are:

• use a hash for the file path, this keeps the request in one frame on bxCAN, making it more reliable and avoids the symlink
• allows target node to be specified