Support Arduino Nano

[neilenns]

Fixes #579

This adds support for trying different baud rates when flashing a given board. It is primarily intended to resolve the "which baud rate should we use for a Nano" problem, but is done in a general fashion that could apply to any device that uses avrdude to flash.

Changes include:

1. Version 7.0 of avrdude to get the -x attempts command line parameter. (I pulled the x86 version of avrdude). We need this for nano flashing so we can reduce the attempts from 10 to 1. Otherwise Nano flash failure takes way, way, way too long.
2. New TimeoutForFirmwareUpdate property in the .board.json file to remove a hard-coded timeout value I missed in the original .board.json implementation
3. New Attempts property in the .board.json file to control how many times avrdude should attempt connections to a board
4. Change BaudRate property to BaudRates (an array) in the .board.json files
5. Schema updates to reflect the property additions/changes
6. New Migrate() method on the Board object to ensure any random old .board.json files that might exist will still work
7. Update RunAvrDude() to use the Attempts property and to loop over the BaudRates until one works. This really is the meat of the change to make this all work and is pretty small.

The baud rates are attempted in the order they are listed in the .board.json file which makes it easy for us to prioritize a given baud rate. For the Nano we'll likely want to list 115200 first and 57600 second assuming the new, fast, bootloader is more common.

Changes tested by @elral (both old and new Nanos) and Jamie (new Nano) and me (new Nano).

[DocMoebiuz]

Really cool change. Thanks!

[DocMoebiuz]

I think it would make sense to include the nano board json for which this is all done and then we could also call #579 done.

I also would propose to set all other boards' retry value to 1 because we don't expect that a retry is required. So if it fails, it can fail fast

[neilenns]

Json added, updated retries on the other boards.

[neilenns]

Once this merges we'll still need to update the firmware builds to build the nano firmware, update the vs project to include it in desktop builds, and I think that's it?

[DocMoebiuz]

Once this merges we'll still need to update the firmware builds to build the nano firmware, update the vs project to include it in desktop builds, and I think that's it?

Yeah, why not do that and then we can include the .hex file right away with this PR? and then this is really the complete nano support.

[DocMoebiuz]

i really would like to merge it but without the nano hex file it feels incomplete.

[neilenns]

Nano firmware PR: MobiFlight/MobiFlight-FirmwareSource#223

[DocMoebiuz]

Mercy boocoo!