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>

tests/crypto/mbedtls_psa/prj.conf

@@ -3,4 +3,3 @@ CONFIG_ZTEST=y
 
 CONFIG_MBEDTLS=y
 CONFIG_MBEDTLS_PSA_CRYPTO_C=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y

tests/crypto/mbedtls_psa/testcase.yaml

@@ -11,8 +11,6 @@
 # - no TF-M enabled devices because we assume that the TF-M implementation
 #   of PSA crypto is working fine on the platforms that support TF-M.
 # - platform should be testable by the CI.
-# - enable CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG in order to reduce as much
-#   as possible usage of legacy modules in Mbed TLS.
 # - pick 1 platform which supports entropy driver and 1 which does not. The
 #   latter case will allow to test
 #   CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG_ALLOW_NON_CSPRNG.
@@ -34,8 +32,11 @@ tests:
     # Pick a platform which does not have an entropy driver. In this case we
     # enable the timer random generator because it's always available on all
     # platforms.
+    # Explicitly select CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG because this is
+    # not "automatically selected" when there is no CSPRNG available.
     integration_platforms:
       - qemu_x86
     extra_configs:
+      - CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
       - CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG_ALLOW_NON_CSPRNG=y
       - CONFIG_TEST_RANDOM_GENERATOR=y

tests/crypto/secp256r1/mbedtls.conf

@@ -1,5 +1,4 @@
 CONFIG_MBEDTLS=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
 CONFIG_MBEDTLS_PSA_CRYPTO_C=y
 CONFIG_MBEDTLS_PSA_P256M_DRIVER_ENABLED=y
 

tests/crypto/secp256r1/p256-m_raw.conf

@@ -1,5 +1,4 @@
 CONFIG_MBEDTLS=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
 CONFIG_MBEDTLS_PSA_CRYPTO_C=y
 CONFIG_MBEDTLS_PSA_P256M_DRIVER_ENABLED=y
 CONFIG_MBEDTLS_PSA_P256M_DRIVER_RAW=y