mk-ca-bundle: added SHA-384 signature algorithm

Certificates based on SHA-1 are being phased out[1].
So we should expect a rise in certificates based on SHA-2.
Adding SHA-384 as a valid signature algorithm.

[1] https://blog.mozilla.org/security/2014/09/23/phasing-out-certificates-with-sha-1-based-signature-algorithms/

Signed-off-by: Bruno Thomsen <bth@kamstrup.dk>