Follow up to Mbed-TLS/TF-PSA-Crypto#307. Clean up things that have to be done in a separate subsequent step because the work is spread over multiple repositories.

• Remove remaining traces MBEDTLS_PLATFORM_GET_ENTROPY_ALT from:
  1. mbedtls;
  2. mbedtls-framework;
  3. TF-PSA-Crypto.
• Add a test to validate that an NV-seed-only configuration requires MBEDTLS_ENTROPY_NO_SOURCES_OK. Requires check_config.h: move to core and test TF-PSA-Crypto#387.