test: Improve handling of sending commands

We expect commands to be echoed and this should happen quite quickly,
since U-Boot is sitting at the prompt waiting for a command.

Reduce the timeout for this situation. Try to produce a more useful
error message when something goes wrong. Also handle the case where the
connection has gone away since the last command was issued.

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