LED usage throughout the library

[fgb]

There are very few LEDs on our boards and we've recently started toggling them during the normal operation of some modules (e.g. radio). The usage of LEDs within traps (e.g. battery) makes a bit more sense, although we should always allow a project to easily override this (i.e. by setting its own callback).

I suggest we stop controlling LEDs from library modules, so that a particular user/project can freely make use of all LEDs. When developing a module, though, you can feel free to pepper it with LED toggles, but these should be removed once a module is committed as stable.

Let me know what you think.

cc. @Humhu, @ryanjulian, @abuchan, @dhaldane, @apullin, @ronf-ucb.