Add u2f(4), a HID driver for FIDO/U2F security keys

[wulf]

Summary

This driver is a stripped-down version of hidraw(4). It implements subset of commands required for FIDO/U2F keys only.

User-visible changes are:
It attaches to FIDO/U2F top level collection. So it can be automagically loaded with devd
It sets 660 root:u2f mode to device node. No need in special devd scripts
It emulates USB_GET_DEVICEINFO ioctl

[manu]

I guess this review should be splitted in multiple ones.
There is the addition of u2f(4) yes but a lot of stuff is changed in the hid code too.