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>

cmd/cramfs.c

@@ -13,6 +13,7 @@
 #include <common.h>
 #include <command.h>
 #include <env.h>
+#include <image.h>
 #include <malloc.h>
 #include <mapmem.h>
 #include <linux/list.h>