batman-adv-specific parts are moved to a new package
gluon-status-page-mesh-batman-adv.

This package only provides the choice between "security" and "performance
mode". It should only be enabled when specified in site.mk.

To reduce the number of packages that need to be listed in
GLUON_SITE_PACKAGES, this adds a new variable GLUON_FEATURES. Sets of
packages are enabled automatically based on the combination of listed
feature flags.

Site-specified package feeds can provide their own feature flag
definitions.