esp8266: Change platform name from ESP8266 to esp8266.

The port name is lowercase, and this change is made for consistency with
the docs and other ports.

esp8266/mpconfigport.h

@@ -152,6 +152,6 @@ extern const struct _mp_obj_module_t onewire_module;
 #define MICROPY_MPHALPORT_H "esp_mphal.h"
 #define MICROPY_HW_BOARD_NAME "ESP module"
 #define MICROPY_HW_MCU_NAME "ESP8266"
-#define MICROPY_PY_SYS_PLATFORM "ESP8266"
+#define MICROPY_PY_SYS_PLATFORM "esp8266"
 
 #define _assert(expr) ((expr) ? (void)0 : __assert_func(__FILE__, __LINE__, __func__, #expr))