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/common/update.c b/common/update.c
index bfb760e..c8dd346 100644
--- a/common/update.c
+++ b/common/update.c
@@ -8,6 +8,7 @@
 
 #include <common.h>
 #include <cpu_func.h>
+#include <image.h>
 
 #if !(defined(CONFIG_FIT) && defined(CONFIG_OF_LIBFDT))
 #error "CONFIG_FIT and CONFIG_OF_LIBFDT are required for auto-update feature"