riotboot: allow to overwrite slot 1
riotboot: allow to overwrite slot 1

By erasing slot 1 header the slot gets invalidated.
This is very useful while debugging, since we can
force the bootloader to ignore anything on that
slot.

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