The current elf loader uses too much memory. For example, I have a
executable with a bss section of 400 MB and I set the ram size to 512
MB. Qemu uses about 780MB of RAM (which is fine), but there's a peak at
1.6 GB during initialization (this is not fine).

This patch fixes two things:
 1) do not allocate each elf program twice.
 2) do not allocate memory for areas that are only zeros.

For this we need a new field in Rom: "datasize" which is the size of the
allocated data. If datasize is less than romsize, it means that the area
from datasize to romsize is filled with zeros.
Signed-off-by: NFabien Chouteau <chouteau@adacore.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>