gnrc_netdev: add deprecation note

[miri64]

Fixes #4975.

[kaspar030]

Hm, OK, maybe just something like "This interface is deprecated. Use netdev2 for new drivers."? Anyhow, any note is fine, so change if you feel like, ACK otherwise. :)

[miri64]

I feel like some explanation of why you should do something is always helpful ;-)

[kaspar030]

Well, this way someone might come to the conclusion that he only wants to support gnrc and then get sad when he realizes we won't merge his code. :)

[miri64]

Deprecated is deprecated, so if someone uses it regardless, because they did not read the whole note, its their own fault ;-)

[OlegHahm]

I second @kaspar030. Maybe you can just reverse the order of the sentences, i.e. first say that netdev2 is the current way to go, then explain what was the shortcoming of the old interface. (I'm not so sure that the explanation is needed at all, but I don't have strong feelings here.)

[miri64]

Done.

[haukepetersen]

ACK when squashed

[miri64]

Squashed