Add core stack headers into bacdef.h and cleanup includes. (#602)

* Added dependent BACnet stack headers into bacdef.h file.

* Changed bacdef.h and other stack includes in c/h files to have a common pattern.

* Moved bits.h, bytes.h, and bacnet_stack_exports.h under bacnet/basic/sys/ folder.

src/bacnet/bacstr.c

@@ -38,19 +38,10 @@
 #include <string.h>
 #include <limits.h>
 #include <ctype.h>
-#include "bacnet/config.h"
+/* BACnet Stack defines - first */
+#include "bacnet/bacdef.h"
+/* BACnet Stack API */
 #include "bacnet/bacstr.h"
-#include "bacnet/bits.h"
-
-/* TODO: For some reason my Zephyr build for non-native targets does not
- *       see a definition for strnlen(), but it is visible in when
- *       compiling for native_posix. This results in the compiler
- *       emitting a warning, forcing Zephyr's sanitycheck() script to stop.
- *       Until this is chased down, the definition is being provided here.
- */
-#if __ZEPHYR__ && !CONFIG_NATIVE_APPLICATION
-size_t strnlen(const char *, size_t);
-#endif
 
 /** @file bacstr.c  Manipulate Bit/Char/Octet Strings */
 #ifndef BACNET_STRING_UTF8_VALIDATION