core/or1k-elf-loader: use libelf instead of or1k-elf-objdump

This patch removes the dependency on or1k-elf-objdump which
was used to extract the payload of the input ELF file.

It now uses libelf (from elfutils) which is available in
most of the current Linux distributions.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>