command: No need to disable irqs in sendf reentrant check

As long as the code is careful when writing the in_sendf variable it
should be safe to update it without having to disable irqs.

Also, make sure in_sendf is cleared on shutdown.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/avr/serial.c

@@ -141,6 +141,7 @@ console_get_output(uint8_t len)
 void
 console_push_output(uint8_t len)
 {
+    barrier();
     writeb(&transmit_max, readb(&transmit_max) + len);
     enable_tx_irq();
 }