Improve programs/cert_write with a way to set the signature digest
Improve programs/cert_write with a way to set the signature digest

This is useful for generating SHA-1 and MD5 certificates for test
purposes.  I guess RSA-PSS could be added too, but I don't need that
now.

Signed-off-by: Dave Rodgman <c7ff43d67af1dd23561452e7410f0b0fcc98e527@arm.com>

ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls