netdev: Provide maximum value of netdev_opt_t for further definitions
Merge pull request #1774 from authmillenon/netdev-opt-max

netdev: Provide maximum value of netdev_opt_t for further definitions
drivers: Make general IEEE 802.15.4 driver part of netdev
[SQUASH ME] netdev.802154: add capability to get default address mode for sending
[SQUASH ME] cc2420: adapt NETDEV_OPT_SRC_LEN