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/lib/lmb.c b/lib/lmb.c
index 7936477..676b3a0 100644
--- a/lib/lmb.c
+++ b/lib/lmb.c
@@ -153,7 +153,7 @@
 	arch_lmb_reserve(lmb);
 	board_lmb_reserve(lmb);
 
-	if (IMAGE_ENABLE_OF_LIBFDT && fdt_blob)
+	if (CONFIG_IS_ENABLED(OF_LIBFDT) && fdt_blob)
 		boot_fdt_add_mem_rsv_regions(lmb, fdt_blob);
 }