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/net.c b/net/net.c
index b9bf0d1..d8a60b6 100644
--- a/net/net.c
+++ b/net/net.c
@@ -93,6 +93,7 @@
 #include <env.h>
 #include <env_internal.h>
 #include <errno.h>
+#include <image.h>
 #include <net.h>
 #include <net/fastboot.h>
 #include <net/tftp.h>