common: Move the image globals into image.h

These three globals relate to image handling. Move them to the image
header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/nand.c b/cmd/nand.c
index 92089a7..4de6892 100644
--- a/cmd/nand.c
+++ b/cmd/nand.c
@@ -20,6 +20,7 @@
  */
 
 #include <common.h>
+#include <image.h>
 #include <linux/mtd/mtd.h>
 #include <command.h>
 #include <console.h>