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>

cmd/disk.c

@@ -6,6 +6,7 @@
 #include <common.h>
 #include <command.h>
 #include <cpu_func.h>
+#include <image.h>
 #include <part.h>
 
 int common_diskboot(cmd_tbl_t *cmdtp, const char *intf, int argc,