gluon-mesh-batman-adv-core: keep disabled state even when the mesh interface section name is changed

Is makes sense to always look for both ibss_radio* and mesh_radio* sections
to determine if the meshing should be enabled when regenerating these
sections. Doing this, the disabled state will survive updates changing the
section name (either updating from pre-2015.2 while keeping IBSS, or
changing from IBSS to 11s or vice-versa).

If both ibss_radio* and mesh_radio* sections exist, the disabled state will
be kept correctly for each section, the behaviour is changed only when
creating a section that didn't exist before.

Fixes #549