mbedtls: use CSPRNG whenever possible as PSA random source

The main problem of MBEDTLS_PSA_CRYPTO_LEGACY_RNG is that it
brings in some legacy modules (entropy + ctr_drbg/hmac_drbg)
which means extra ROM/RAM footprint.
MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG instead simply calls to the
CSPRNG which makes it definitely smaller.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>