HAL_Empty: allow return of last value in RCOutput

this is needed for linux boards with no native RCOutput that use SBUS
out on a serial port to ensure they can display the servo values in
the GCS

libraries/AP_HAL_Empty/RCOutput.cpp

@@ -1,5 +1,6 @@
 
 #include "RCOutput.h"
+#include <AP_Math/AP_Math.h>
 
 using namespace Empty;
 
@@ -18,12 +19,23 @@ void RCOutput::disable_ch(uint8_t chan)
 {}
 
 void RCOutput::write(uint8_t chan, uint16_t period_us)
-{}
+{
+    if (chan < ARRAY_SIZE(value)) {
+        value[chan] = period_us;
+    }
+}
 
-uint16_t RCOutput::read(uint8_t chan) {
+uint16_t RCOutput::read(uint8_t chan)
+{
+    if (chan < ARRAY_SIZE(value)) {
+        return value[chan];
+    }
     return 900;
 }
 
 void RCOutput::read(uint16_t* period_us, uint8_t len)
-{}
+{
+    len = MIN(len, ARRAY_SIZE(value));
+    memcpy(period_us, value, len*sizeof(value[0]));
+}
 

libraries/AP_HAL_Empty/RCOutput.h

@@ -13,4 +13,6 @@ class Empty::RCOutput : public AP_HAL::RCOutput {
     void     read(uint16_t* period_us, uint8_t len) override;
     void     cork(void) override {}
     void     push(void) override {}
+private:
+    uint16_t value[16];
 };