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>
diff --git a/arch/arc/lib/bootm.c b/arch/arc/lib/bootm.c
index 41408c2..ed6c5df 100644
--- a/arch/arc/lib/bootm.c
+++ b/arch/arc/lib/bootm.c
@@ -63,7 +63,7 @@
 	       "(fake run for tracing)" : "");
 	bootstage_mark_name(BOOTSTAGE_ID_BOOTM_HANDOFF, "start_kernel");
 
-	if (IMAGE_ENABLE_OF_LIBFDT && images->ft_len) {
+	if (CONFIG_IS_ENABLED(OF_LIBFDT) && images->ft_len) {
 		r0 = 2;
 		r2 = (unsigned int)images->ft_addr;
 	} else {