esp32: Support building with ESP IDF 4.0-beta1.

This commit adds support for a second supported hash (currently set to the
4.0-beta1 tag).  When this hash is detected, the relevant changes are
applied.

This allows to start using v4 features (e.g. BLE with Nimble), and also
start doing testing, while still supporting the original, stable, v3.3 IDF.

Note: this feature is experimental, not well tested, and network.LAN and
network.PPP are currently unsupported.

ports/esp32/network_ppp.c

@@ -26,6 +26,7 @@
  * THE SOFTWARE.
  */
 
+#if !MICROPY_ESP_IDF_4
 #include "py/runtime.h"
 #include "py/mphal.h"
 #include "py/objtype.h"
@@ -282,3 +283,5 @@ const mp_obj_type_t ppp_if_type = {
     .name = MP_QSTR_PPP,
     .locals_dict = (mp_obj_dict_t*)&ppp_if_locals_dict,
 };
+
+#endif // !MICROPY_ESP_IDF_4