Copter: add short delay to arming to allow RC input

The short delay gives time for the RC inputs to be processed which
removes the chance of a false-positive on the "late frame" radio check.
A false positive could lead to an immediate disarm right after arming.

ArduCopter/motors.pde

@@ -197,6 +197,9 @@ static void init_arm_motors()
     sprayer.test_pump(false);
 #endif
 
+    // short delay to allow reading of rc inputs
+    delay(30);
+
     // enable output to motors
     output_min();