Right now 'ipv6_addr_split_iface' assumes that the interface specifier
will always be a number (based on GNRC way of identifying interfaces),
but this may not be always the case.In order to be able to use the
Network Interface API, interfaces should be referred by their name.
This changes 'ipv6_addr_split_iface' so it returns a pointer to the
string that specifies the interface.
4d4f6ae1d6