fix(core1-control): Free semaphore between ready waiting and loading

Up to now, the semaphore was locked in core1_wait_ready() and later
freed in core1_load().  This is problematic when only core1_wait_ready()
is called (e.g. due to an invalid payload).

Instead, free the semaphore before leaving core1_wait_ready() and
reaquire it in core1_load().  This way an incorrect call sequence cannot
lead to a lockup.

Ref: #228

epicardium/api/control.c

@@ -227,6 +227,7 @@ void core1_wait_ready(void)
 		 * above loop, waiting for a new IVT.
 		 */
 		if (core1_info.ready) {
+			SEMA_FreeSema(_CONTROL_SEMAPHORE);
 			break;
 		}
 
@@ -245,6 +246,9 @@ void core1_wait_ready(void)
 
 void core1_load(void *ivt, char *args)
 {
+	while (SEMA_GetSema(_CONTROL_SEMAPHORE) == E_BUSY) {
+	}
+
 	/* If the core is currently in an API call, reset it. */
 	API_CALL_MEM->call_flag = _API_FLAG_IDLE;
 	API_CALL_MEM->id        = 0;