vboot_signer: Add kernel subkey required for signing kernel keyblock

Currently, the keyblock was being generated using
data_key.vbprivk. However, we need to use kernel_subkey.vbprivk for
kernel keyblock generation. This did not create any issues until now
because dev-mode just throws a message saying that keyblock is
invalid. But, normal-mode does not boot if keyblock is
invalid. Add extra parameter for passing in kernel subkey to
vboot_signer script.

TEST="make bootimage-nodeps" generates correctly signed
boot.img. Verified that the image boots fine in normal mode.

Change-Id: I0fc2183b466e34ddf1d98c9532072548504fcec4
Signed-off-by: Furquan Shaikh <furquan@google.com>
diff --git a/core/product.mk b/core/product.mk
index 4e8bff1..e97cba4 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -104,6 +104,7 @@
     PRODUCT_SYSTEM_PROPERTY_BLACKLIST \
     PRODUCT_SYSTEM_SERVER_JARS \
     PRODUCT_VBOOT_SIGNING_KEY \
+    PRODUCT_VBOOT_SIGNING_SUBKEY \
     PRODUCT_VERITY_SIGNING_KEY \
     PRODUCT_SYSTEM_VERITY_PARTITION \
     PRODUCT_VENDOR_VERITY_PARTITION \