cpu/atmega_common: make RAM available as heap
cpu/atmega_common: make RAM available as heap

Fills __malloc_heap_end with the current SP before it enters thread mode to make the remaining RAM available as a heap.

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