netdev.tests: Add fixes for environment variables

+ minor include fix
netdev.tests: Kill processes properly
cpu: Implement SPI driver for sam3x8e
boards: Implement SPI definitions for arduino-due
Merge pull request #1778 from authmillenon/netdev-test-env-fixes

netdev.tests: Add fixes for environment variables
Merge pull request #1584 from PeterKietzmann/add_sam3x_spi

cpu/board: updated  spi driver for arduino-due