lib: Add ASN.1 encoder

We have a need in the TPM2 trusted keys to return the ASN.1 form of the TPM
key blob so it can be operated on by tools outside of the kernel.  The
specific tools are the openssl_tpm2_engine, openconnect and the Intel
tpm2-tss-engine.  To do that, we have to be able to read and write the same
binary key format the tools use.  The current ASN.1 decoder does fine for
reading, but we need pieces of an ASN.1 encoder to write the key blob in
binary compatible form.

For backwards compatibility, the trusted key reader code will still accept
the two TPM2B quantities that it uses today, but the writer will only
output the ASN.1 form.

The current implementation only encodes the ASN.1 bits we actually need.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>