0.7.0 - a second board, and a release for each

Until now there was one board and one firmware image. Now there are two boards, and that turns out to change more than a file name.
The ESP32-P4 comes in two silicon families, and an image built for one will not start on the other - the header carries both a minimum and a maximum revision. The Waveshare ESP32-P4-ETH is an older revision; the Function EV board is rev 3.2. So they cannot share an image, and both have to be built.
What that means in practice:
- every board is built by CI and gets its own release files, with the board name in the file name;
- every board gets its own update manifest, so a device only ever checks for images that will run on it;
- the plain files stay where they were, so devices flashed before this release keep updating exactly as they did.
A board is described by a small settings overlay rather than by code, which is what makes adding the next one cheap.