lr-wpan: Add a LrWpanNetDeviceBase for lrwpan netdevices extensions

The present MR introduces the LrWpanNetDeviceBase abstract class to allow the introduction of new netdevices or variations of LrWpanNetDevices.

Furthermore, the GetMac() function has been modified (in both the newly introduced LrWpanNetDeviceBase and the now descendent LrWpanNetDevice) to return LrWpanMacBase (An abstraction of lr-wpan compliant Mac layer). The LrWpanMacBase was previously in the code base but it was underutilized. This change will allow for any LrWpanMacBase descendants to be used by the NetDevice. This will open the door to access variations of compliant lr-wpan MAC layers within the same NetDevice.

All this combined will reduce the code base dependency on LrWpanMac and LrWpanNetDevice, which becomes important for protocols that run on top of this lr-wpan technology (e.g. sixlowpan & the soon to be introduced in the code base Zigbee protocol).