x86: add proper terminal for qemu-i386

This includes GNU readline features and debugging.

Build with `make BOARD=qemu-i386 all-debug`.
Run with `make BOARD=qemu-i386 term`.

Debug with `make BOARD=qemu-i386 debug`. The default debugger is `gdb`.
Also supported are `debug-tui` (GDB Text User Interface), `debug-kdbg`,
and `debug-ddd`. Set a breakpoint in e.g. "startup" or "main", and
hit/write "continue".

The debugger can only run with a quite new toolchain (e.g. Debian
testing). Ubuntu 13.10. for example will likely report a crash in GDB
when switching from 16bit code to 32bit code.