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/net/nfs.c b/net/nfs.c
index 5226e2c..72e1018 100644
--- a/net/nfs.c
+++ b/net/nfs.c
@@ -29,6 +29,7 @@
 #include <common.h>
 #include <command.h>
 #include <flash.h>
+#include <image.h>
 #include <net.h>
 #include <malloc.h>
 #include <mapmem.h>