sandbox: Allow Ctrl-C to work in sandbox

It is useful for Cltl-C to be handled by U-Boot as it is on other boards.
But it is also useful to be able to terminate U-Boot with Ctrl-C.

Add an option to enable signals while in raw mode, and make this the
default. Add an option to leave the terminal cooked, which is useful for
redirecting output.

Signed-off-by: Simon Glass <sjg@chromium.org>