 drivers/cc2420: Implemented missing drop feature
 drivers/cc2420: Implemented missing drop feature

The netdev_driver_t::recv implementation of the cc2420 does not provide the drop
feature. This commit adds it.

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