image: Drop IMAGE_ENABLE_OF_LIBFDT

Add a host Kconfig for OF_LIBFDT. With this we can use
CONFIG_IS_ENABLED(OF_LIBFDT) directly in the tools build, so drop the
unnecessary indirection.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>