Integrate meta-seconorth-distro/fng-install-postinstall

Commit: seco-ne/yocto/layers/meta-seconorth-distro@b1b47267

fng-system: Remove kernel after combine step

Keeping the original kernel together with the combined kernel (kernel + devicetree) wastes space and isn't needed.

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Commit: seco-ne/yocto/layers/meta-seconorth-distro@894bb005

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@a2b67812

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@83f74a24

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