i.MX6 legacy transitional boot-script from 2013 to 2018

This transitional boot-scripts allows booting systems using uEnv.txt or boot.scr targeting the old v2013 based U-Boot for i.MX6.
It must be installed to the boot partition at the root level, explicitly not named boot.scr - but rather legacyboot.scr.

This paragraph duplicates section "Transitioning from 2013.04 and earlier" from our developer documentation at.