Sending CTRL-C to QEMU's stdin aborts the process, even if stdin is being
used as a serial port (at least in the raspi2 machine with "qemu -serial
stdin"). Avoid sending CTRL-C to U-Boot to prevent it exiting.

I'd originally used CTRL-C to make sure that if the character used to
abort autoboot ended up being treated as part of a command as well, it'd
abort command entry and return the prompt to a known state. However, this
is not needed, since aborting the autoboot eats the character used to do
that.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Simon Glass <sjg@chromium.org>