Integrate meta-guf-distro/fng-install-cleanups

Commit: https://gitlab.com/SECO-Northern-Europe/yocto/layers/meta-guf-distro/-/commit/7afa4cf97571814e2dc3c965a78f6c4bc02afb74

fng-install: Fixes for wait_for_mmc, shellcheck/ check if fuser exists

Shellcheck complained about few things. Fuser is not available in FNGSystem 15, sleep instead.

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Commit: https://gitlab.com/SECO-Northern-Europe/yocto/layers/meta-guf-distro/-/commit/b12398a44f505f1f27e88c958e6ad4280b4d9ded

fng-install: Remove md5sum implementation

The md5sums where only be used to adapt the contents of a possibly available checksum file to the changes done during installation. Removed this, as it has not been tested a long time and not been used much. If needed again, this should go to a fngpostinstallation step.

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Commit: https://gitlab.com/SECO-Northern-Europe/yocto/layers/meta-guf-distro/-/commit/59c6f0fe9af09ace934f2e43fe218176016f6b47

fng-install: Remove --OS option to install kernel separate from image

This option was not correctly implemented anymore, and not used either.

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Commit: https://gitlab.com/SECO-Northern-Europe/yocto/layers/meta-guf-distro/-/commit/775175bd0ddfbec37dcc23213c9cb15f22fd2f2b

fng-install: Comments, improve --dry mode, little cleanup