riotboot: update documentation to match automatic use
riotboot: update documentation to match automatic use

FLASHFILE is now set to RIOTBOOT_EXTENDED_BIN, changing the meaning of
make all`, `flash`, `flash-only`.
This commits updates the documentation accordingly.

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