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/loader.c b/boot/bootutil/src/loader.c
index 95a1572..a328172 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -38,6 +38,10 @@
 #define BOOT_LOG_LEVEL BOOT_LOG_LEVEL_INFO
 #include "bootutil/bootutil_log.h"
 
+#ifdef APP_mynewt
+#include "mynewt/config.h"
+#endif
+
 #define BOOT_MAX_IMG_SECTORS        120
 
 /** Number of image slots in flash; currently limited to two. */