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>

net/tftp.c

@@ -10,6 +10,7 @@
 #include <command.h>
 #include <efi_loader.h>
 #include <env.h>
+#include <image.h>
 #include <mapmem.h>
 #include <net.h>
 #include <net/tftp.h>