use the NULL section instead of offset == 0
use the NULL section instead of offset == 0

This change makes it possible to use the regular API to build up
elf files which include the file header.

serge1/ELFIO,serge1/ELFIO,niosHD/ELFIO,niosHD/ELFIO,niosHD/ELFIO,serge1/ELFIO,niosHD/ELFIO