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_validate.c b/boot/bootutil/src/image_validate.c
index 9413d31..00d43c6 100644
--- a/boot/bootutil/src/image_validate.c
+++ b/boot/bootutil/src/image_validate.c
@@ -28,6 +28,10 @@
 #include "bootutil/sha256.h"
 #include "bootutil/sign_key.h"
 
+#ifdef APP_mynewt
+#include "mynewt/config.h"
+#endif
+
 #ifdef MCUBOOT_SIGN_RSA
 #include "mbedtls/rsa.h"
 #endif