Merged in merlin17/nuttx/xbee (pull request #484)

drivers/wireless:  Adds XBee S2C (802.15.4 firmware) support. XBee driver emulates mac802154 interface

* drivers/wireless/xbee: Adds xbee_netdev. Very similar to mac802154_netdev

* configs/same70-xplained: Starts adding support for XBee radio

* drivers/wireless/ieee802154/xbee: More structuring of XBee driver

* drivers/wireless/ieee802154/xbee: More Xbee work. Starts adding support on Clicker2

* drivers/wireless/ieee802154/xbee: More XBee MAC code

* configs/clicker2-stm32: More work to add XBee radio support

* drivers/wireless/ieee802154/xbee: Most of driver is now structured. No build errors

* configs/clicker2-stm32: Adjustments to XBee click module support

* drivers/xbee: Changes md_ prefix to xd_ prefix for xbeenet_driver_s fields

* drivers/xbee: XBee network device now passes MAC events via IOCTL

* drivers/xbee: Support querying device for parameters, setting parameters, and structures association/startpan logic

* configs/clicker2-stm32: Fixes Xbee lower half ATTN poll logic

* drivers/xbee: Removes dependce on CONFIG_IEEE802154_NETDEV

* drivers/xbee: Supports MAC RESET.request primitive

* drivers/xbee: Exposes generic AT query to the rest of driver

* drivers/xbee: Sets local values when writing setting change to Xbee device

* drivers/xbee: Finish association logic

* drivers/xbee: Adds xbee_get_mhrlen( ) allocating enough space for API frame header to frame

* drivers/xbee: Finishes transmit/receive logic

* drivers/xbee: Fixes xbee_netdev to match logic in mac802154_netdev.c

* drivers/xbee: Rearranges logic to prevent a loop condition where recursion could potentially occur to the point of deadlocking the system

Approved-by: Gregory Nutt <gnutt@nuttx.org>