loader: fdt: Add fdt_is_setup function

When efi_autoload is called it will call fdt_setup_fdtp which setup the
dtb and overlays. If a user already loaded at dtb or overlays or just
printed the efi provided dtb, this will re-setup everything and also
re-applying the overlays.
Test that everything is setup before doing it again.

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D20059