Fix broken Mynewt BOOTUTIL_ -> MCUBOOT_ mapping

Fix mapping of syscfg.yml BOOTUTIL_* vars to C #defines, which was broken
on most .c files for lacking inclusion of mynewt "config.h"
diff --git a/boot/bootutil/src/image_ec256.c b/boot/bootutil/src/image_ec256.c
index 615fb9f..59cda9e 100644
--- a/boot/bootutil/src/image_ec256.c
+++ b/boot/bootutil/src/image_ec256.c
@@ -19,6 +19,10 @@
 
 #include <string.h>
 
+#ifdef APP_mynewt
+#include "mynewt/config.h"
+#endif
+
 #ifdef MCUBOOT_SIGN_EC256
 #include "bootutil/sign_key.h"