esp32: Add initial support for ESP32S2 SoCs.

Builds against IDF v4.3-beta2.

Signed-off-by: Damien George <damien@micropython.org>
diff --git a/ports/esp32/esp32_ulp.c b/ports/esp32/esp32_ulp.c
index 50244cd..8e4ce9c 100644
--- a/ports/esp32/esp32_ulp.c
+++ b/ports/esp32/esp32_ulp.c
@@ -26,6 +26,8 @@
 
 #include "py/runtime.h"
 
+#if CONFIG_IDF_TARGET_ESP32
+
 #include "esp32/ulp.h"
 #include "esp_err.h"
 
@@ -95,3 +97,5 @@
     .make_new = esp32_ulp_make_new,
     .locals_dict = (mp_obj_t)&esp32_ulp_locals_dict,
 };
+
+#endif // CONFIG_IDF_TARGET_ESP32