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/board/synopsys/hsdk/hsdk.c b/board/synopsys/hsdk/hsdk.c
index 892b94b..226fbba 100644
--- a/board/synopsys/hsdk/hsdk.c
+++ b/board/synopsys/hsdk/hsdk.c
@@ -871,7 +871,7 @@
 	if (env_common.core_mask.val == ALL_CPU_MASK)
 		return 0;
 
-	if (!IMAGE_ENABLE_OF_LIBFDT || !images->ft_len) {
+	if (!CONFIG_IS_ENABLED(OF_LIBFDT) || !images->ft_len) {
 		pr_err("WARN: core_mask setup will work properly only with external DTB!\n");
 		return 0;
 	}