This PR adds a more CLI flag, so that the les-server can serve light clients even the local node is not synced yet.
This functionality is needed in some testing environments(e.g. hive). After launching the les server, no more blocks will be imported so the node is always marked as "non-synced".
This moves the eth config definition into a separate package, eth/ethconfig.
Packages eth and les can now import this common package instead of
importing eth from les, reducing dependencies.
Co-authored-by: Felix Lange <fjl@twurst.com>
