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/net.c b/cmd/net.c
index 6bce214..1b724cc 100644
--- a/cmd/net.c
+++ b/cmd/net.c
@@ -10,6 +10,7 @@
 #include <common.h>
 #include <command.h>
 #include <env.h>
+#include <image.h>
 #include <net.h>
 
 static int netboot_common(enum proto_t, cmd_tbl_t *, int, char * const []);