distro/pxeboot: Handle prompt variable

Regarding the documentation found here:
https://github.com/u-boot/u-boot/blob/master/common/menu.c#L347



If both timeout and prompt is set to 0 the default entry shall
be booted immediately. However the current behaviour is that
the prompt is shown (tested with distroboot) until the user
selects an entry (no timeout).

This change implements a behaviour as documented. It was tested
with distroboot.

Signed-off-by: Manuel Traut <manuel.traut@mt.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>