Replace occurences of 'ktn'/'KTN' with 'ked'/'KED'

In the future we will reference the BSP as meta-ked-bsp. Adjust names of layers, files, etc. accordingly. Only some places like Linux and U-Boot recipe, repository and branch names are keeping the 'ktn' for now.

KED stands for "Kontron Electronics Deutschland GmbH" where the BSP is maintained. Nowadays it support only products of the KED branch and products of the "Kontron Europe GmbH" or other branches are not supported.