esp_system: introduce system time functions

- Introduce system time function and concept of system time provider.
esp_timer is system time provider when present.
- Set the reference point for system time, g_startup_time.
- Use the system time functions in newlib instead of calling esp_timer
functions directly

components/esp_system/include/esp_private/startup_internal.h

@@ -64,6 +64,10 @@ static __attribute__((used)) esp_system_init_fn_t _SECTION_ATTR_IMPL(".esp_syste
 static __attribute__((used)) __VA_ARGS__ void __esp_system_init_fn_##f(void) // [refactor-todo] this can be made public API if we allow components to declare init functions, 
                                                                              // instead of calling them explicitly
 
+extern uint64_t g_startup_time;   // Startup time that serves as the point of origin for system time. Should be set by the entry 
+                                  // function in the port layer. May be 0 as well if this is not backed by a persistent counter, in which case
+                                  // startup time = system time = 0 at the point the entry function sets this variable.
+
 #ifdef __cplusplus
 }
 #endif