netifd: Make interface identifier of delegated IPv6 address configurable

netifd: Make interface identifier of delegated IPv6 address configurable

The ip6ifaceid UCI interface parameter makes the interface identifier of the delegated IPv6 address configurable.
The parameter can have the following values:
    eui64 : Interface identifier is generated from the interface's MAC address
    random : Interface identifier is generated randomly
    fixed value : Interface identifier is a fixed value (eg ::1:2)

The latter is the default value with a fixed value of ::1 for backwards compatibility

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
Signed-off-by: Joeri Barbarien <joeri.barbarien@gmail.com>