mkimage: Move ARRAY_SIZE to header file

Move this definition from aisimage.c to mkimage.h so that it is available
more widely.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Marek Vasut <marex@denx.de>
diff --git a/tools/mkimage.h b/tools/mkimage.h
index ea45f5c..e07a615 100644
--- a/tools/mkimage.h
+++ b/tools/mkimage.h
@@ -42,6 +42,8 @@
 #define debug(fmt,args...)
 #endif /* MKIMAGE_DEBUG */
 
+#define ARRAY_SIZE(x)		(sizeof(x) / sizeof((x)[0]))
+
 #define MKIMAGE_TMPFILE_SUFFIX		".tmp"
 #define MKIMAGE_MAX_TMPFILE_LEN		256
 #define MKIMAGE_DEFAULT_DTC_OPTIONS	"-I dts -O dtb -p 500"