Fix interference between PSA volatile keys and built-in keys
when MBEDTLS_PSA_CRYPTO_BUILTIN_KEYS is enabled and
MBEDTLS_PSA_KEY_SLOT_COUNT is more than 4096. This overlap used to make it
possible that a volatile key would receive the identifier of a built-in key,
and is now caught by a static assertion.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
