kernel: make CONFIG_BUILD_ARM_APPENDED_DTB_IMAGE optional

We have kernel images (picoimx) that do not want a legacy appended DTB,
so move this config to recommended so that it can be overridden by a
product kernel config fragment.

Bug: 28768869

Change-Id: Ife7c27636c2da520a15d430fb5aedf535cfe3d3f
4 files changed