fngsystem: add script to run postinstallation on installed yocto

This is a fix for issue with ID 746-001246: "FNG-System: Add way to manually update the configuration"

To solve the problem (to be able to update the settings in Yoto after xconfig import), we added a fngsystem-rootfs-postinstall.sh script to the FNG System. It is a simplified version of the existing fng-install.sh script and right now it supports only an "--AB-target" argument from the original script.

This script can be run like this:

sh /usr/share/yocto-install/fngsystem-rootfs-postinstall.sh

Note: due to lack of iMX8 boards for which AB partition layout may be used, this functionality in the script has not been tested yet (but already exists).