scripts: coredump: coredump_gdbserver in pipeline

Add support to coredump_gdbserver.py for running in a pipeline,
communicating through stdin/stdout instead of a socket.

This allows starting it from inside gdb, with:

    target remote | coredump_gdbserver.py --pipe <elf_file>  <log_file>

Signed-off-by: Kevin ORourke <kevin.orourke@ferroamp.se>