bvb: Update path to key since repo was moved.

We recently moved Brillo verified boot from system/bvb to external/bvb
so update the path accordingly.

TEST=Build with 'BOARD_BVB_ENABLE := true' succeeds.
BUG=29099910

Change-Id: I66f3b90e23d6b2afc09f81af571ede7b573325e9
diff --git a/core/Makefile b/core/Makefile
index 79a0dc3..064af66 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -518,7 +518,7 @@
 ifndef BOARD_BVB_KEY_PATH
 # If key path isn't specified, use the 4096-bit test key.
 INTERNAL_BVBTOOL_SIGN_BOOT_IMAGE_ARGS := --algorithm SHA256_RSA4096 \
-	--key system/bvb/test/testkey_rsa4096.pem
+	--key external/bvb/test/testkey_rsa4096.pem
 else
 INTERNAL_BVBTOOL_SIGN_BOOT_IMAGE_ARGS := \
 	--algorithm $(BOARD_BVB_ALGORITHM) --key $(BOARD_BVB_KEY_PATH)