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 >=0.152 (from elfutils) which is available in
most of the current Linux distributions.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Add mmuart core
or1200-generic: Add UART output for verilator simulations

The wb_bfm based UART model is not synthesizable and does not
work with verilator. Use the real uart16550 core instead for
verilator simulations and decode the serial stream with an mmuart
that displays the value