netdev.tests: Add fixes for environment variables

+ minor include fix
netdev.tests: Kill processes properly
Merge pull request #1778 from authmillenon/netdev-test-env-fixes

netdev.tests: Add fixes for environment variables
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