Added helper methods for checking derivation policy flag

Signed-off-by: Samuel Bailey <samuel.bailey@arm.com>

psa-crypto/src/types/key.rs

@@ -142,6 +142,20 @@ impl Attributes {
         }
     }
 
+    /// Check if a key has permissions to be derived from
+    pub fn is_derivable(self) -> bool { self.policy.usage_flags.derive }
+
+    /// Check derive permission of a fallible way
+    pub fn can_derive_from(self) -> Result<()> {
+        if self.is_derivable() {
+            Ok(())
+        }
+        else {
+            error!("Key attributes do not permit derivation.");
+            Err(Error::NotPermitted)
+        }
+    }
+
     /// Check if the alg given for a cryptographic operation is permitted to be used with the key
     pub fn is_alg_permitted(self, alg: Algorithm) -> bool {
         match self.policy.permitted_algorithms {