Simplify process_wait()

The function process_wait() returned a bool (true if the process
terminated successfully) and provided the exit code via an output
parameter exit_code.

But the returned value was always equivalent to exit_code == 0, so just
return the exit code instead.

app/src/file_handler.c

@@ -178,7 +178,7 @@ file_handler_stop(struct file_handler *file_handler) {
         if (!process_terminate(file_handler->current_process)) {
             LOGW("Could not terminate install process");
         }
-        process_wait(file_handler->current_process, NULL);
+        process_wait(file_handler->current_process); // ignore exit code
         file_handler->current_process = PROCESS_NONE;
     }
     mutex_unlock(file_handler->mutex);