esp32/boards: Add board definition for Olimex ESP32 PoE boards.

The major setting is about the PHY interface configuration.  The
configuration matches the Olimex ESP32 Gateway as well.

Tested with esp-idf v4.2.4 and Olimex ESP32 POE boards.

ports/esp32/boards/OLIMEX_ESP32_POE/board.json

@@ -0,0 +1,24 @@
+{
+    "deploy": [
+        "../deploy.md"
+    ],
+    "docs": "",
+    "features": [
+        "BLE",
+        "WiFi",
+        "MicroSD",
+        "Battery Charging",
+        "Ethernet",
+        "PoE",
+        "Breadboard friendly"
+    ],
+    "id": "esp32",
+    "images": [
+        "ESP32-POE-ISO-1.jpg"
+    ],
+    "mcu": "esp32",
+    "product": "ESP32",
+    "thumbnail": "",
+    "url": "https://www.olimex.com/",
+    "vendor": "Olimex"
+}

ports/esp32/boards/OLIMEX_ESP32_POE/board.md

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+The following files are daily firmware for Olimex ESP32 boards with Ethernet.
+They match the boards ESP32 ETH-PoE, ESP32 ETH-PoE-ISO and ESP32 Gateway.
+
+This firmware is compiled using ESP-IDF v4.x.

ports/esp32/boards/OLIMEX_ESP32_POE/mpconfigboard.cmake

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+set(SDKCONFIG_DEFAULTS
+    boards/sdkconfig.base
+    boards/sdkconfig.ble
+    boards/OLIMEX_ESP32_POE/sdkconfig.board
+)

ports/esp32/boards/OLIMEX_ESP32_POE/mpconfigboard.h

@@ -0,0 +1,2 @@
+#define MICROPY_HW_BOARD_NAME "Olimex ESP32 ETH"
+#define MICROPY_HW_MCU_NAME "ESP32"

ports/esp32/boards/OLIMEX_ESP32_POE/sdkconfig.board

@@ -0,0 +1,4 @@
+CONFIG_ETH_PHY_INTERFACE_RMII=y
+CONFIG_ETH_RMII_CLK_OUTPUT=y
+CONFIG_ETH_RMII_CLK_OUT_GPIO=17
+CONFIG_LWIP_LOCAL_HOSTNAME="ESP32_POE"