Bump MbedTLS to 2.12.0

[nalimilan]

Version 2.6.0 has a security issue, see https://tls.mbed.org/tech-updates/security-advisories/mbedtls-security-advisory-2018-02.

Another option would be to move to the long-term supported version, 2.7.5. This would have the advantage that we will be able to backport security fixes in the future without risking to break anything.

[cdluminate]

As a side note, Debian unstable uses mbedtls 2.12.0, with which julia was built.

[KristofferC]

CI wasn't too hot when this was opened so let's rerun it.

[KristofferC]

Seems to fail on the mac builder:

configure: error: one or more libs available at link-time are not available run-time. Libs used at link-time: -lssh2 -lssh2 -lmbedtls -lmbedx509 -lmbedcrypto 
make[2]: *** [scratch/curl-7.56.0/build-configured] Error 1
make[1]: *** [julia-deps] Error 2

[nalimilan]

Not sure what to do about that OS X error. I've tried using the most recent CURL, which includes fixes when building using MbedTLS, and using the older MbedTLS 2.7.5, and it's still there...

[PallHaraldsson]

There by now https://tls.mbed.org/tech-updates/releases/mbedtls-2.13.0-2.7.6-and-2.1.15-released with new features for 2.13.0; ans security update for it and 2.7.6

[KristofferC]

#30618