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/load.c b/cmd/load.c
index bed973e..fab30fe 100644
--- a/cmd/load.c
+++ b/cmd/load.c
@@ -13,6 +13,7 @@
 #include <cpu_func.h>
 #include <env.h>
 #include <flash.h>
+#include <image.h>
 #include <s_record.h>
 #include <net.h>
 #include <exports.h>