Fix warn in overwrite-only with slot0 validation

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index ab8ac7b..d7b08f6 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -43,7 +43,7 @@
 
 static struct boot_loader_state boot_data;
 
-#ifdef MCUBOOT_VALIDATE_SLOT0
+#if defined(MCUBOOT_VALIDATE_SLOT0) && !defined(MCUBOOT_OVERWRITE_ONLY)
 static int boot_status_fails = 0;
 #define BOOT_STATUS_ASSERT(x)                \
     do {                                     \