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>