AP_HAL: support accessors for dshot_period_us

add support for selectively disabling serial LEDs and passthrough
provide mutator to allow iomcu to set dshot rate
support HAL_SERIAL_ESC_COMM_ENABLED and DISABLE_SERIAL_ESC_COMM
update dshot docs
make default serial led enablement dependent on dshot

libraries/AP_HAL/AP_HAL_Boards.h

@@ -283,6 +283,22 @@
 #define HAL_DSHOT_ALARM_ENABLED 0
 #endif
 
+#ifndef HAL_DSHOT_ENABLED
+#define HAL_DSHOT_ENABLED 1
+#endif
+
+#ifndef HAL_SERIALLED_ENABLED
+#define HAL_SERIALLED_ENABLED HAL_DSHOT_ENABLED
+#endif
+
+#ifndef HAL_SERIAL_ESC_COMM_ENABLED
+#ifdef DISABLE_SERIAL_ESC_COMM
+#define HAL_SERIAL_ESC_COMM_ENABLED 0
+#else
+#define HAL_SERIAL_ESC_COMM_ENABLED 1
+#endif
+#endif
+
 #ifndef HAL_HNF_MAX_FILTERS
 // On an F7 The difference in CPU load between 1 notch and 24 notches is about 2%
 // The difference in CPU load between 1Khz backend and 2Khz backend is about 10%
@@ -316,10 +332,6 @@
 #define HAL_USE_OCTOSPI 0
 #endif
 
-#ifndef HAL_ENABLE_DSHOT
-#define HAL_ENABLE_DSHOT 1
-#endif
-
 #ifndef __RAMFUNC__
 #define __RAMFUNC__
 #endif

libraries/AP_HAL/RCOutput.h

@@ -311,6 +311,12 @@ public:
      */
     virtual void set_dshot_rate(uint8_t dshot_rate, uint16_t loop_rate_hz) {}
 
+    /*
+      Set the dshot period in us, only for use by the IOMCU
+     */
+    virtual void set_dshot_period(uint32_t period_us, uint8_t dshot_rate) {}
+    virtual uint32_t get_dshot_period_us() const { return 0; }
+
     /*
       Set the dshot ESC type
      */
@@ -372,7 +378,7 @@ public:
      * Options are (ticks, percentage):
      * 20/7/14, 35/70
      * 11/4/8, 36/72
-     * 8/3/6, 37/75
+     * 8/3/6, 37/75 <-- this is the preferred duty cycle and has some support on the interwebs
      */
     // bitwidths: 8/3/6 == 37%/75%
     static constexpr uint32_t DSHOT_BIT_WIDTH_TICKS_DEFAULT = 8;