this activates the package by default when using the batman feature
while still allowing to explicitly remove it like this:
GLUON_SITE_PACKAGES := \
-gluon-ebtables-limit-arp
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.
