In attributes, keep track of slot number through a dedicated field

In `psa_key_attributes_t`, keep track of whether `slot_number` has been set
through a dedicated field, rather than using a flag.

This paves the way to removing `flags`, which is not used for anything else.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 71535fa..91f5f16 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1245,7 +1245,7 @@
     const psa_key_attributes_t *attributes,
     psa_key_slot_number_t *slot_number)
 {
-    if (attributes->flags & MBEDTLS_PSA_KA_FLAG_HAS_SLOT_NUMBER) {
+    if (attributes->has_slot_number) {
         *slot_number = attributes->slot_number;
         return PSA_SUCCESS;
     } else {