Bugfix: LB_DERIVATIVE was used the wrong way around
When LB_DERIVATIVE is FALSE, only LB_PARENT_DEBIAN_INSTALLER_DISTRIBUTION should be used. Derivatives can use both LB_PARENT_DEBIAN_INSTALLER_DISTRIBUTION and LB_DEBIAN_INSTALLER_DISTRIBUTION.
When LB_DERIVATIVE is FALSE, only LB_PARENT_DEBIAN_INSTALLER_DISTRIBUTION should be used. Derivatives can use both LB_PARENT_DEBIAN_INSTALLER_DISTRIBUTION and LB_DEBIAN_INSTALLER_DISTRIBUTION.