Release notes

20170620-nitrogen-4.1.15_2.0.0_ga-jessie
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   - apt update/upgrade on rootfs to make it up to date
   - kernel upgrade to ABI=21 , commit 506aafc, "imx-wm8960: skip matching check"
   - apt cache has been cleared
   - sgtl-on alias has been changed
   - 6x_bootscript has been modified
   - the following files have been removed to eliminate incrementing network device letters:
      /etc/udev/rules.d/70-persistent-net.rules
      /lib/udev/rules.d/75-persistent-net-generator-rules