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/elf.c b/cmd/elf.c
index 57736f9..ba06df0 100644
--- a/cmd/elf.c
+++ b/cmd/elf.c
@@ -18,6 +18,7 @@
 #include <cpu_func.h>
 #include <elf.h>
 #include <env.h>
+#include <image.h>
 #include <net.h>
 #include <vxworks.h>
 #ifdef CONFIG_X86