Integrate meta-seconorth-distro/cherry_pick_fng_install

Commit: seco-ne/yocto/layers/meta-seconorth-distro@74b33419

fng-install: Fixed post-install for fngsystem

The post-install wasn't handling the alternative boot configuration. Also the combine step for kernel + devicetree was failing on i.MX8 devices because of space limitations.

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Commit: seco-ne/yocto/layers/meta-seconorth-distro@9b022e2b

fng-install: Updated post-install scripts

The environmental variable based passing of devicetrees and overlays has been replaced entirely by a file-based mechanism. The documentation, comments, and format of the post-install scripts have been extended and reworked.

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Commit: seco-ne/yocto/layers/meta-seconorth-distro@92399b95

fng-install: Clarify interface between fng-install and postinstall steps