AP_HAL: add millis16 to HAL; provide weak implementation

This wraps every minute or so - but if you only need to track intervals
under ~65 seconds could save RAM

libraries/AP_HAL/system.cpp

@@ -0,0 +1,6 @@
+#include "system.h"
+
+uint16_t WEAK AP_HAL::millis16()
+{
+    return millis() & 0xFFFF;
+}

libraries/AP_HAL/system.h

@@ -14,6 +14,7 @@ void panic(const char *errormsg, ...) FMT_PRINTF(1, 2) NORETURN;
 
 uint32_t micros();
 uint32_t millis();
+uint16_t millis16();
 uint64_t micros64();
 uint64_t millis64();