[crypto] Disable MD5 as an OID-identifiable algorithm by default

[crypto] Disable MD5 as an OID-identifiable algorithm by default

Disable the use of MD5 as an OID-identifiable algorithm.  Note that
the MD5 algorithm implementation will still be present in the build,
since it is used implicitly by various cryptographic components such
as HTTP digest authentication; this commit removes it only from the
list of OID-identifiable algorithms.

It would be appropriate to similarly disable the use of SHA-1 by
default, but doing so would break the use of OCSP since several OCSP
responders (including the current version of openca-ocspd) are not
capable of interpreting the hashAlgorithm field and so will fail if
the client uses any algorithm other than the configured default.

Signed-off-by: Michael Brown <mcb30@ipxe.org>