Draft: Include booloader-related recipes and configuration snippets

We want to use the meta-ked-bootloader repo to build and manage bootloader software. We also want to include a Yocto layer in the repo that contains recipes and configuration. This layer can be used by BSPs to include all bootloader-related metadata.

As we are creating kind of a cyclic dependency between metadata from the BSP repository that is used for building the bootloader and metadata from the bootloader repository that is used to build the BSP, we reference a feature branch for the BSP for now. After everything has landed we can revert this to the main branch.

See here for the WIP branch for the BSP repo that this MR is referencing.