Release notes

20160913-nitrogen-4.1.15_1.2.0_ga-xenial
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   - apt update/upgrade on rootfs to make it up to date
   - kernel upgrade to ABI=6 , commit 768b84f, "ft5x06_ts: add release_timer to workaround firmware bug"
   - apt cache has been cleared
   - fix fstab error in MATE image
   - added gzipped image variant for those who use gzwrite to create eMMC image
   - partition size was modified from 3680MB to 3600MB to fit eMMC