sha1: add dedicated config option

The sha1 code is currently compiled for everyone, but in reality, it's
only used by the FIT code.  So make it optional just like MD5.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/include/image.h b/include/image.h
index d2e78fe..dcf3a7b 100644
--- a/include/image.h
+++ b/include/image.h
@@ -56,6 +56,7 @@
 #include <libfdt.h>
 #include <fdt_support.h>
 #define CONFIG_MD5		/* FIT images need MD5 support */
+#define CONFIG_SHA1		/* and SHA1 */
 #endif
 
 /*