ng_netconf: adapt byte orders to output

[miri64]

The byte order of addresses in ng_netdev/ng_netconf is a little bit messed up:

• The shell output and configuration it's unclear: For Ethernet it's network byte order, though the doc states it's in host byte order, for ng_at86rf2xx it is little endian (regardless of the host byte order), because
• for sending ng_at86rf2xx assumes the address is stored in little endian
• I don't even know know what xbee is doing.

My solution: addresses (including PAN ID) shall always be configured in network byte (big endian) order. If the protocol does not use network byte order (e.g. IEEE 802.15.4) it needs to convert it to their specific order.

[miri64]

Need to rethink this one.