devtool: sdk-update: fix traceback without update server set

devtool: sdk-update: fix traceback without update server set

If the SDK update server hasn't been set in the config (when building
the extensible SDK this would be set via SDK_UPDATE_URL) and it wasn't
specified on the command line then we were failing with a traceback
because we didn't pass the default value properly - None is interpreted
as no default, meaning raise an exception if no such option exists.

Additionally we don't need the try...except anymore either because with
a proper default value, NoSectionError is caught as well.

(From OE-Core master rev: 9763c1b83362f8445ed6dff2804dd7d282861f79)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>