Revert to the previous version where the return value of uart_getenv()

is being ignored. It's optional and the lack of environment variable
is not an error condition.

sys/arm/at91/uart_cpu_at91rm9200usart.c

@@ -80,5 +80,6 @@ uart_cpu_getdev(int devtype, struct uart_devinfo *di)
 	uart_bus_space_io = &at91_bs_tag;
 	uart_bus_space_mem = NULL;
 	/* Check the environment for overrides */
-	return (uart_getenv(devtype, di, class));
+	uart_getenv(devtype, di, class);
+	return (0);
 }