stm32f2/Makefile.include: add riotboot requirements
stm32f2/Makefile.include: add riotboot requirements

- stm32f2 uses sectors instead of pages, they go from 16KB to
  128KB. Smaller sectors are at the begining of the flash. Slots
  must start at the begining of a sector to not overlap.
- Minimum required RIOBOOT_HDR_LEN or stm32f2 is 0x200
  to respect vector table alignment

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