additional board def for 16MB ethernet

the main difference is that "esp32-poe" is used as variant, instead of "esp32dev". Seems that this is needed to define a few special purpose pins on ethernet boards.

Might help to fix https://github.com/MoonModules/WLED/issues/23

boards/esp32_16MB-poe.json

@@ -0,0 +1,38 @@
+{
+"build": {
+    "arduino": {
+    "ldscript": "esp32_out.ld"
+    },
+    "core": "esp32",
+    "extra_flags": "-DARDUINO_ESP32_DEV -DARDUINO_ESP32_POE",
+    "f_cpu": "240000000L",
+    "f_flash": "40000000L",
+    "flash_mode": "dio",
+    "mcu": "esp32",
+    "variant": "esp32-poe",
+    "partitions": "partitions_16M.csv"
+},
+"connectivity": [
+    "wifi",
+    "bluetooth",
+    "ethernet",
+    "can"
+],
+"debug": {
+    "openocd_board": "esp-wroom-32.cfg"
+},
+"frameworks": [
+    "arduino",
+    "espidf"
+],
+"name": "ESP32 16MB",
+"upload": {
+    "flash_size": "16MB",
+    "maximum_ram_size": 327680,
+    "maximum_size": 16777216,
+    "require_upload_port": true,
+    "speed": 2000000
+},
+"url": "https://en.wikipedia.org/wiki/ESP32",
+"vendor": "Multiple"
+}