Extended radio api and AT86RF231 implementation

[rousselk]

This PR completes the implementation of the AT86RF231 radio driver; the latter now implements all of the netdev_802154_driver_t features (that were until now only implemented as stubs), and adds some new options to this interface:

• NETDEV_OPT_CAN_MONITOR indicates whether the transceiver can go into "monitor" (a.k.a.: "promiscuous") mode;
• NETDEV_OPT_CCA_THRESHOLD for getting or setting the threshold value that is used by a transceiver to determine if radio channel is busy (CCA);
• NETDEV_OPT_SFD_COUNT for getting and resetting the number of SFDs (start of frame delimiters) detected by the transceiver: I think it is a good indicator to help estimate the traffic on the radio medium;
  The two last options are planned to be used in future MAC protocol implementations.

[rousselk]

Note that this enhanced driver has been tested with success on FIT IoT-LAB's M3 nodes.

[jnohlgard]

How do I test this? Can I use the rpl_udp example? Do I need any particular commands in the rpl shell?

[rousselk]

A new 'sfdcnt' shell command has been added in the shell (sys/shell/commands/sc_transceiver.c, see last commit).

[OlegHahm]

should stay disabled by default

[OlegHahm]

Since @thomaseichinger is woring on the driver, too, I guess it makes sense if he reviews.

[PeterKietzmann]

@rousselk @haukepetersen @bapclenet does it make sense to keep this PR open?

[PeterKietzmann]

(just because of #2825)

[haukepetersen]

For some reason I was un-aware of this PR. As far as I see it, it contains some functionality that is not included in the remodeled ng_at.. driver. So I say lets keep this open and try to merge the functionality to the new driver.

[biboc]

@rousselk, do you think you will have time to adapt your changes to the new at86rf2xx driver? Or should someone else do it.

[OlegHahm]

@bapclenet, @daniel-k, @thomaseichinger, any volunteers to port these changes to the new driver?

[daniel-k]

I'm too busy right now. Maybe sometime later if these features proof useful to me, but I need to get other things done at the moment, sorry.

[biboc]

The same as daniel-k, I'm too busy at the moment, I hope I will be able to work on RIOT soon.

[rousselk]

I have just upgraded to current RIOT codebase (from 2014.12 release !).
This branch is now just irrelevant. I'm closing it, and will create a new branch/PR when ready.

[biboc]

Great! Thank you @rousselk