passing back the unused memory pointer

[Rogelio Serrano]

Can we pass the pointer to the first unused memory to the kernel being 
booted by GRUB? That way i have to avoid reading all the pointers from 
the multiboot struct to look for the first usable memory after 
everything to avoid overwriting anything important. maybe we push it 
into the bootstack before booting or extend the multiboot_info structto 
include this pointer.