cpu/samd5x: make sure RIOTBOOT_LEN is 2*FLASHPAGE_SIZE
cpu/samd5x: make sure RIOTBOOT_LEN is 2*FLASHPAGE_SIZE

The flashpage size on samd5x is 8k, so set RIOTBOOT_LEN accordingly.

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