Allow the kernel to use FDT DTBs directly from hardware without loader
assistance.

Print the source of the DTB we are using.
Add an FDT_DTB_STATIC_ONLY option.

This option forces the use of a static DTB and not check for DTB
provided by the loader or in ROM.  This allows the static FDT to act as a
fallback for defective hardware in the normal case while also supporting
developers who need to change the FDT.