openssl/lk: Enable signed boot img check

This uses openssl to check the boot image prior
to jumping to kernel to see if it is a signed
kernel image.

Change-Id: I92927fd9317a0f701dab395cc6d39929c64340c1
diff --git a/target/init.c b/target/init.c
index 9dbcc9d..3da8d89 100644
--- a/target/init.c
+++ b/target/init.c
@@ -91,3 +91,12 @@
 {
 	return 0;
 }
+
+__WEAK bool target_use_signed_kernel(void)
+{
+#if _SIGNED_KERNEL
+	return 1;
+#else
+	return 0;
+#endif
+}