Follow the same pattern that was used in the past to remove dependency
on MBEDTLS_RSA_C and use PSA_WANT instead.
Relying on MBEDTLS_RSA_C is fine only when builtin drivers are compiled
since all PSA_WANT are converted to legacy build symbols. However when
builtin drivers are not built (ex: in case of TF-M), then part of the code
in TLS/X509 won't be compiled because MBEDTLS_RSA_C is not set. OTOH
it's not possible to declare that symbol in a configuration file because
it's a legacy one and it will be rejected by buildtime checks.
Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
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