tests/riotboot: use FLASHFILE for the generated file
tests/riotboot: use FLASHFILE for the generated file

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.

jasonatran/RIOT,mtausig/RIOT,toonst/RIOT,ant9000/RIOT,rfuentess/RIOT,mfrey/RIOT,lazytech-org/RIOT,kYc0o/RIOT,x3ro/RIOT,kaspar030/RIOT,josephnoir/RIOT,aeneby/RIOT,miri64/RIOT,lazytech-org/RIOT,OlegHahm/RIOT,ant9000/RIOT,josephnoir/RIOT,yogo1212/RIOT,smlng/RIOT,rfuentess/RIOT,toonst/RIOT,OTAkeys/RIOT,A-Paul/RIOT,OTAkeys/RIOT,cladmi/RIOT,authmillenon/RIOT,OlegHahm/RIOT,josephnoir/RIOT,jasonatran/RIOT,x3ro/RIOT,x3ro/RIOT,basilfx/RIOT,authmillenon/RIOT,A-Paul/RIOT,A-Paul/RIOT,kaspar030/RIOT,toonst/RIOT,yogo1212/RIOT,basilfx/RIOT,rfuentess/RIOT,A-Paul/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,yogo1212/RIOT,authmillenon/RIOT,aeneby/RIOT,OTAkeys/RIOT,cladmi/RIOT,ant9000/RIOT,rfuentess/RIOT,OlegHahm/RIOT,josephnoir/RIOT,jasonatran/RIOT,lazytech-org/RIOT,RIOT-OS/RIOT,smlng/RIOT,jasonatran/RIOT,smlng/RIOT,miri64/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,authmillenon/RIOT,mfrey/RIOT,mtausig/RIOT,basilfx/RIOT,mfrey/RIOT,mtausig/RIOT,kYc0o/RIOT,OTAkeys/RIOT,toonst/RIOT,yogo1212/RIOT,authmillenon/RIOT,OTAkeys/RIOT,ant9000/RIOT,kaspar030/RIOT,yogo1212/RIOT,josephnoir/RIOT,cladmi/RIOT,A-Paul/RIOT,kYc0o/RIOT,ant9000/RIOT,cladmi/RIOT,kaspar030/RIOT,smlng/RIOT,miri64/RIOT,x3ro/RIOT,basilfx/RIOT,smlng/RIOT,toonst/RIOT,mtausig/RIOT,lazytech-org/RIOT,OlegHahm/RIOT,rfuentess/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,jasonatran/RIOT,lazytech-org/RIOT,x3ro/RIOT,aeneby/RIOT,authmillenon/RIOT,miri64/RIOT,aeneby/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,mfrey/RIOT,kYc0o/RIOT,miri64/RIOT,basilfx/RIOT,aeneby/RIOT,mtausig/RIOT,mfrey/RIOT,cladmi/RIOT