Include the *_sha1.i files in their own *_codedigest.c files.

This way we do not need to rebuild util.c and/or config.c whenever
any unrelated source file in src/common or src/or has changed.

src/common/util_codedigest.c

@@ -0,0 +1,11 @@
+
+#include "util.h"
+
+const char *
+libor_get_digests(void)
+{
+  return ""
+#include "common_sha1.i"
+    ;
+}
+