uVisor: Update the RPC header files

This follows:

ARMmbed/uvisor#02ecf05 - "Remove unused TFN_RPC_Callback"
ARMmbed/uvisor#483ddda - "rpc: Add `rpc_fncall_wait`"
ARMmbed/uvisor#fdb7dce - "rpc: Add missing UVISOR_EXTERN to
                          rpc_fncall_waitfor declaration"

features/FEATURE_UVISOR/includes/uvisor/api/inc/rpc.h

@@ -33,7 +33,6 @@
 typedef uint32_t uvisor_rpc_result_t;
 
 typedef uint32_t (*TFN_Ptr)(uint32_t, uint32_t, uint32_t, uint32_t);
-typedef int (*TFN_RPC_Callback)(int);
 
 /** Wait for incoming RPC.
  *
@@ -43,6 +42,21 @@ typedef int (*TFN_RPC_Callback)(int);
  * @param timeout_ms   specifies how long to wait (in ms) for an incoming RPC
  *                     message before returning
  */
-int rpc_fncall_waitfor(const TFN_Ptr fn_ptr_array[], size_t fn_count, uint32_t timeout_ms);
+UVISOR_EXTERN int rpc_fncall_waitfor(const TFN_Ptr fn_ptr_array[], size_t fn_count, uint32_t timeout_ms);
+
+/** Wait for an outgoing RPC to finish.
+ *
+ * Wait for the result of a previously started asynchronous RPC. After this
+ * call, ret will contain the return value of the RPC. The return value of this
+ * function may indicate that there was an error or a timeout with non-zero.
+ *
+ * @param result[in]     The token to wait on for the result of an asynchronous RPC
+ * @param timeout_ms[in] How long to wait (in ms) for the asynchronous RPC
+ *                       message to finish before returning
+ * @param ret[out]       The return value resulting from the finished RPC to
+ *                       the target function
+ * @returns              Non-zero on error or timeout, zero on successful wait
+ */
+UVISOR_EXTERN int rpc_fncall_wait(uvisor_rpc_result_t result, uint32_t timeout_ms, uint32_t * ret);
 
 #endif /* __UVISOR_API_RPC_H__ */