We cannot add the equivalent of
cmake_package_install yet as the build in
tf-psa-crypto still references some headers
in ./include/mbedtls like mbedtls_config.h.
We cannot add the equivalent of
cmake_subproject yet as currently only
the case of the Mbed TLS parent project is
supported.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>