ELF: allow using program headers and physical addresses to build the image

[Spaqin]

If you're doing something less common, like memory overlays, with multiple sections sharing the same virtual address, the default behavior of iterating over section headers doesn't work - data gets overwritten, corrupting the program.

load doesn't work in this case as it's just a part of an ELF that needs to be loaded in a specific spot, following its physical address (and would feel rather hacky regardless). I didn't want to touch the section headers for compatibility, leaving the virtual addresses case untouched, so I added a new attribute: elf_use_ph.

With that flag enabled, the program data will be copied using program header data at the physical address within.

Example usage:

the_ROM_image:
{
   [bootloader]szl.elf
   top.bit
   [elf_use_ph]runtime.elf
}