command: Check for reentrant calls to sendf()

Allow sendf() to be called from irq and timer context - check for the
case where sendf() is called while already in sendf() and simply
discard those messages.  This makes it safe to use output() debugging
calls even in irq and timer context.

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

docs/Protocol.md

@@ -105,9 +105,7 @@ output("The value of %u is %s with size %u.", x, buf, buf_len);
 ```
 
 The output() function is similar in usage to printf() - it is intended
-to generate and format arbitrary messages for human consumption. It is
-a wrapper around sendf() and as with sendf() it should not be called
-from interrupts or timers.
+to generate and format arbitrary messages for human consumption.
 
 Declaring constants
 -------------------