mbedtls: update to 2.16.7

mbedtls: update to 2.16.7

Mbed TLS 2.16.7 is a maintenance release of the Mbed TLS 2.16 branch,
and provides bug fixes and minor enhancements. This release includes
fixes for security issues and the most severe one is described in more
detail in a security advisory:
https://tls.mbed.org/tech-updates/security-advisories/mbedtls-security-advisory-2020-07

* Fix a side channel vulnerability in modular exponentiation that could
reveal an RSA private key used in a secure enclave.
* Fix side channel in mbedtls_ecp_check_pub_priv() and
mbedtls_pk_parse_key() / mbedtls_pk_parse_keyfile() (when loading a private
key that didn't include the uncompressed public key), as well as
mbedtls_ecp_mul() / mbedtls_ecp_mul_restartable() when called with a NULL
f_rng argument. An attacker with access to precise enough timing and
memory access information (typically an untrusted operating system
attacking a secure enclave) could fully recover the ECC private key.
* Fix issue in Lucky 13 counter-measure that could make it ineffective when
hardware accelerators were used (using one of the MBEDTLS_SHAxxx_ALT
macros).

Due to Mbed TLS moving from ARMmbed to the Trusted Firmware project, some
changes to the download URLs are required. For the time being, the
ARMmbed/mbedtls Github repository is the canonical source for Mbed TLS.

Signed-off-by: Magnus Kroken <mkroken@gmail.com>