Implement NETOPT_ACK_REQ configuration option.

[aeneby]

This fixes #5287 - currently the option NETOPT_AUTOACK is incorrectly being used to set the AR (acknowledge request) bit of the 802.15.4 MAC header. This PR introduces a new option for doing this instead, NETOPT_ACK_REQ, leaving it up to the driver to set AUTOACK when requested.

[thomaseichinger]

This change definitely matches the intended option to set better. Did you check with all the other occurrences of NETDEV_AUTOACK they actually mean 'autoack'?

[aeneby]

Yes, it's a bit of a mess unfortunately. For example, the kw2xrf and the at86rf2xx drivers seem to do a combination of enabling both AUTOACK and the AR bit when the NETOPT_AUTOACK option is set. But at least this patch constrains the problem to the drivers themselves, and introduces a new option so that the problem can be fixed - we can open new PRs for the drivers themselves afterwards.

[PeterKietzmann]

@aeneby I agree with your proposal. This PR won't change the driver behaviour (should be cleared up in following PRs) but introduces capabilities to differentiate between a hardware procedure on receiving frames and to set the regarding bit in the 802.15.4 header on sending frames.

[PeterKietzmann]

@aeneby you need to add an entry for the new option here. Once done, we're not far from merging this PR

[aeneby]

Thanks @PeterKietzmann - done, rebased and squashed.

[PeterKietzmann]

ACK and go

[jfischer-no]

added option to kw2xrf -> #5469