ext: mbedtls: Enable easy integration with POSIX API

If CONFIG_POSIX_API is defined, automatically use time() function.
The alternative to that is to explicitly configure mbedTLS' timing
source via API, as required for "raw code", but if user enables
POSIX API, we as well might use existing mbedTLS integration.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>