API change in netdev2

Hi, quick note to all network device driver and MAC protocol developers: The API of netdev2 was changed slightly [1] in that the event_callback now doesn’t expect a void * argument anymore. This argument has proven to be superfluous, since the data was always stored in the netdev2_t struct anyway. As a consequence this member of the struct was renamed from isr_arg to context.

If you are working currently on a network device driver implementing netdev2 or on a MAC protocol utilizing netdev2, please rebase!

Thanks for your attention, Martine

[1] https://github.com/RIOT-OS/RIOT/pull/4871