Support flashing with multiple wifi configs

[addshore]

Fixes #158

High level changes:

• New jag commands for interacting with a list of
  wifi settings, rather than just one
• Changed the underlying computer and device config
  structure to support mutliple wifis
• Added a scan to the jag startup to look for wifis
  that actually exist, connecting to ones we can see

This is very much a first iteration that I have confirmed
working on my local device, but I expect it could do with
some tidying, but it works now and is something to look
at!

[addshore]

So, this does change behaviour a little, as I for example had an app that depended on effective := firmware.config["wifi"] to try and figure out if wifi was configured..

  has-wifi-configured -> bool:
    // Check if we have wifi configured (for web server)
    effective := firmware.config["wifi"]
    if effective == null:
      return false
    ssid := effective.get wifi.CONFIG-SSID
    if ssid != null and ssid != "":
      return true
    return false

And that now totally crashes.

This ties into my desire to be able to tell if wifi is configured or connected, without opening it

https://discord.com/channels/918498540232253480/1420350603338584166

[floitsch]

I like it.

[floitsch]

It's if-error now.

Suggested change

	timeout := int.parse header --on-error=: null
	timeout := int.parse header --if-error=: null

[floitsch]

I wonder if we can make the PR simpler by using the fact that the jag and the firmware are always of the same version.
As such, we can transition (without backwards-compatibility) to the new format.
However, we would need to be able to read the old config file (on the desktop). The rest should be able to assume that both sides use the new format.