extension: net: Refactor, fix and improve network extensions

- 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)

extensions/network/config-networkd/netplan/10-dhcp-all-interfaces.yaml

@@ -0,0 +1,16 @@
+# Added by Armbian
+#
+# Reference: https://netplan.readthedocs.io/en/stable/netplan-yaml/
+#
+# Let systemd-networkd manage all Ethernet devices on this system, but be configured by Netplan.
+
+network:
+  version: 2
+  renderer: networkd
+  ethernets:
+    all-eth-interfaces:
+      match:
+        name: "*"
+      dhcp4: yes
+      dhcp6: yes
+      ipv6-privacy: yes # Enabled by default on most current systems, but networkd currently doesn't enable IPv6 privacy by default, see https://man.archlinux.org/man/systemd.network.5