Change firmware flash utility to download latest firmware from github rather than use bundled binaries

[t0mpr1c3]

To be discussed.

[t0mpr1c3]

I suggested elsewhere that we postpone this until after 1.0.

[dl1com]

I suggested elsewhere that we postpone this until after 1.0.

I suggest to keep it as an "optional target" for 1.0. If @X-sam comes up with a working solution, it would be great.
But until this happens (does not matter until or after 1.0), we need a solution for this, as #465 shows. Otherwise people won't be able to use the software.

I've opened #466 with a proposal to handle this until something better appears.

[jonathanperret]

Is this still a blocker for 1.0? The current 1.0 flash dialog is not worse than the 0.95 dialog and a firmware image is now automatically embedded in builds.