Integrate meta-guf-machine/imx8m-plus-default-dtb

Commit: https://gitlab.com/garz-fricke/yocto/layers/meta-guf-machine/-/commit/20374726bc7adc7f7c13837cf7db292e28cba032

bootscript:default devicetree: Select first devicetree as default

KERNEL_DEVICETREE contains a list of devicetrees. Now the first of those is used as default devicetree for the boot script instead of a hardcoded one.