mbedtls: Don't use tf_snprintf if option not defined

mbedtls: Don't use tf_snprintf if option not defined

If `MBEDTLS_PLATFORM_SNPRINTF_ALT` isn't used, the function
`mbedtls_platform_set_snprintf()` isn't defined.

In case a platform uses a different mbed TLS configuration file than
the one provided by the Trusted Firmware, and it doesn't define the
mentioned build option, this will result in a build error.

This patch modifies the initialization code so that
`mbedtls_platform_set_snprintf()` is only used if
`MBEDTLS_PLATFORM_SNPRINTF_ALT` is defined, allowing platforms to use
it or not depending on their needs.

Change-Id: I1d5c86d57e9b2871ba463030bf89210ebec5178e
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>