This variable was originally introduces for the board "Espressobin" (mvebu64): 38db0b55f9
Use a hook in the mvebu64 and mt7623 family config to set ignored devices for NetworkManager instead.
- Rename extensions with "net-*" prefix
- Put the extensions into their own folder
- Split off time sync packages into their own extensions to be able to be used separately
- Put their config files into directories instead of using inline `cat <<- EOF >`
- Move some other NetworkManager related stuff into the extension
- Remove unneeded steps
- Install iproute2 by default on all images (for the `ip` command)
