Avoid commiting configs inbetween to decrease number of flash writes.

Fixes #714.

Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.

gluon-mesh-batman-adv should not be used without firewall to prevent forwarding
of packets from the mesh to the wan interface.

For some reason, the firewall package won't work at all without
kmod-ipt-nathelper.