devtool: sdk-update: tweak command-line handling of updateserver
Get the default value for updateserver from the configuration file and
show it in the help; also only make the parameter optional if it's
specified. This means we can also drop the check in the function as
argparse will then ensure it's specified if there's no config setting.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
