Analogous function to mbedtls_pk_parse_key (write an encrypted string)

Hi @TrinityTonic
Yes, this PR doesn’t include encrypted keys, but it was one of the comments .
Note this PR is quite old and would probably need reworking.
If you would like this PR to go back to focus, I would suggest you ping on this PR.
Note, that as mentioned here, Mbed TLS is now maintained under open governance at TrustedFirmware.org.
Regards,
Ron