nrf52: include riotboot feature when possible
nrf52: include riotboot feature when possible

The riotboot feature is not compatible with the nordic_softdevice_ble
package at the moment

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