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/mpconfigport.h b/ports/esp32/mpconfigport.h
index c9c4b62..8788963 100644
--- a/ports/esp32/mpconfigport.h
+++ b/ports/esp32/mpconfigport.h
@@ -153,7 +153,9 @@
 #define MICROPY_PY_MACHINE_SPI              (1)
 #define MICROPY_PY_MACHINE_SPI_MSB          (0)
 #define MICROPY_PY_MACHINE_SPI_LSB          (1)
+#ifndef MICROPY_HW_ENABLE_SDCARD
 #define MICROPY_HW_ENABLE_SDCARD            (1)
+#endif
 #define MICROPY_HW_SOFTSPI_MIN_DELAY        (0)
 #define MICROPY_HW_SOFTSPI_MAX_BAUDRATE     (ets_get_cpu_frequency() * 1000000 / 200) // roughly
 #define MICROPY_PY_USSL                     (1)