FLASHFILE is now a generated file when doing `make all`.
This prepares also for when flashers will use `FLASHFILE` as a file to
be flashed.
It currently still needs the hack below for openocd and edbg.
This also fixes the issue when building 'riotboot' in docker that was
being built with the host toolchain.
The tests overrides the target all to be tested by the CI.
All the instructions how to use it are in README.md
The test is successful if the image boots and displays
information about the image and running slot.
Co-authored-by: Federico Pellegrin <fede@evolware.org>
