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/pxe_utils.c b/cmd/pxe_utils.c
index ff8b0dc..8b83021 100644
--- a/cmd/pxe_utils.c
+++ b/cmd/pxe_utils.c
@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <env.h>
+#include <image.h>
 #include <malloc.h>
 #include <mapmem.h>
 #include <lcd.h>