statefiles: change ipv4 MAC statefile syntax

statefiles: change ipv4 MAC statefile syntax

This changes the MAC representation in the lease/statefile so that it
reads "aa:bb:cc:dd:ee:ff" instead of "aabbccddeeff".

No functional changes, but this makes it easier to parse the statefile
in rpcd-mod-luci (for reporting the MAC addresses in LuCI).

We should probably change lease->hwaddr to be a struct ether_addr,
to make it clearer that we only support MAC addresses as hwaddr,
but that's a topic for a different PR.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/332
Signed-off-by: Álvaro Fernández Rojas <[email protected]>