Bugfix/zephyr build for minimal ram (#606)

* Removed BACnet objects from ports/zephyr. There should only be datalink and OS related interfaces in OS ports.

* Added Kconfig and bacnet-config.h options in ports/zephyr to keep small footprint for MCUs having less RAM

* Changed ports/zephry for BACnet/IP and date-time with latest zephyr OS

ports/zephyr/datetime-init.c

@@ -12,14 +12,8 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
-#if CONFIG_NATIVE_APPLICATION
-#include <sys/time.h>
-#elif defined(__ZEPHYR__)
-#include <zephyr/posix/sys/time.h>
-#else
-#include <posix/sys/time.h>
-#endif
 #include <time.h>
+#include <sys/time.h>
 #include "bacnet/datetime.h"