elf: Implement elf_get_file_size to determine size of an ELF image

Implement elf_get_file_size to determine the size of an ELF image
that has been loaded into a buffer much larger than the actual size
of the original file. We determine the size by searching for the
farthest offset declared by the ELF headers.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>