Support extern Ethernet (#60)

see espressif/arduino-esp32#6318

Author: Jason2866

configs/defconfig.esp32

@@ -23,6 +23,8 @@ CONFIG_SPIRAM_TRY_ALLOCATE_WIFI_LWIP=y
 CONFIG_SPIRAM_MALLOC_RESERVE_INTERNAL=0
 CONFIG_ESP32_ULP_COPROC_ENABLED=y
 CONFIG_ESP32_XTAL_FREQ_AUTO=y
+CONFIG_ETH_RMII_CLK_OUTPUT=y
+CONFIG_ETH_RMII_CLK_OUT_GPIO=17
 CONFIG_ETH_SPI_ETHERNET_DM9051=y
 CONFIG_ETH_SPI_ETHERNET_W5500=y
 CONFIG_HTTPD_MAX_REQ_HDR_LEN=1024