AP_HAL: added micros16() method

fast method for both 16 and 32 bit timers

libraries/AP_HAL/system.cpp

@@ -5,6 +5,11 @@ uint16_t WEAK AP_HAL::millis16()
     return millis() & 0xFFFF;
 }
 
+uint16_t WEAK AP_HAL::micros16()
+{
+    return micros() & 0xFFFF;
+}
+
 void WEAK AP_HAL::dump_stack_trace()
 {
     // stack dump not available on this platform

libraries/AP_HAL/system.h

@@ -12,6 +12,7 @@ void init();
 
 void panic(const char *errormsg, ...) FMT_PRINTF(1, 2) NORETURN;
 
+uint16_t micros16();
 uint32_t micros();
 uint32_t millis();
 uint16_t millis16();