pru: Request the host not send more than 496 bytes to the pru

Writes over 496 bytes don't fit in a single "rpmsg" page.  Request the
host limit the number of bytes outstanding to avoid getting "write:
(22)Invalid argument" errors.

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

src/pru/main.c

@@ -115,6 +115,9 @@ DECL_INIT(timer_init);
  * Console IO
  ****************************************************************/
 
+// Writes over 496 bytes don't fit in a single "rpmsg" page
+DECL_CONSTANT("RECEIVE_WINDOW", 496 - 1);
+
 // Process any incoming commands
 void
 console_task(void)