Allow setting 'apt update'-specific options

The '--apt-options' setting can only be used for setting options that are understood by all apt features (install, update, etc.) used by live-build. Attempting to enable a feature specific to 'apt update', such as '--error-on=any', will result in a build failure. To allow these options to be set as well, add an '--apt-update-options' setting to 'lb config'.