Clarify psa_get_and_lock_transparent_key_slot_with_policy() purpose

Clarify the purpose of
psa_get_and_lock_transparent_key_slot_with_policy() and
define it even when MBEDTLS_PSA_CRYPTO_SE_C is disabled
for the purpose of static drivers.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 4670a06..d88836b 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -944,16 +944,16 @@
 /** Get a key slot containing a transparent key and lock it.
  *
  * A transparent key is a key for which the key material is directly
- * available, as opposed to a key in a secure element.
+ * available, as opposed to a key in a secure element and/or to be used
+ * by a secure element.
  *
- * This is a temporary function to use instead of
- * psa_get_and_lock_key_slot_with_policy() until secure element support is
- * fully implemented.
+ * This is a temporary function that may be used instead of
+ * psa_get_and_lock_key_slot_with_policy() when there is no opaque key support
+ * for a cryptographic operation.
  *
  * On success, the returned key slot is locked. It is the responsibility of the
  * caller to unlock the key slot when it does not access it anymore.
  */
-#if defined(MBEDTLS_PSA_CRYPTO_SE_C)
 static psa_status_t psa_get_and_lock_transparent_key_slot_with_policy(
     mbedtls_svc_key_id_t key,
     psa_key_slot_t **p_slot,
@@ -965,7 +965,7 @@
     if( status != PSA_SUCCESS )
         return( status );
 
-    if( psa_key_slot_is_external( *p_slot ) )
+    if( psa_key_lifetime_is_external( (*p_slot)->attr.lifetime ) )
     {
         psa_unlock_key_slot( *p_slot );
         *p_slot = NULL;
@@ -974,11 +974,6 @@
 
     return( PSA_SUCCESS );
 }
-#else /* MBEDTLS_PSA_CRYPTO_SE_C */
-/* With no secure element support, all keys are transparent. */
-#define psa_get_and_lock_transparent_key_slot_with_policy( key, p_slot, usage, alg )   \
-    psa_get_and_lock_key_slot_with_policy( key, p_slot, usage, alg )
-#endif /* MBEDTLS_PSA_CRYPTO_SE_C */
 
 psa_status_t psa_remove_key_data_from_memory( psa_key_slot_t *slot )
 {