decouple failsafe from the rest of the system.

Note: the pwm_common driver has a dependency on the main failsafe code,
the solution is probably to move some of the code into rx_pwm.

src/flight_common.c

@@ -0,0 +1,13 @@
+#include "board.h"
+#include "mw.h"
+
+#include "runtime_config.h"
+
+#include "flight_common.h"
+
+
+void mwDisarm(void)
+{
+    if (f.ARMED)
+        f.ARMED = 0;
+}