esp8266: add support for this chip

Many thanks to cnlohr for the nosdk8266 project:
    https://github.com/cnlohr/nosdk8266

targets/esp8266.json

@@ -0,0 +1,15 @@
+{
+	"inherits": ["xtensa"],
+	"cpu": "esp8266",
+	"build-tags": ["esp8266", "esp"],
+	"linker": "xtensa-esp32-elf-ld",
+	"cflags": [
+		"-mcpu=esp8266"
+	],
+	"linkerscript": "targets/esp8266.ld",
+	"extra-files": [
+		"src/device/esp/esp8266.S"
+	],
+	"binary-format": "esp8266",
+	"flash-command": "esptool.py --chip=esp8266 --port {port} write_flash 0x00000 {bin} -fm qio"
+}