Rename mg_ -> miot_ under fw

`MG_ENABLE_...` macros are not yet renamed because it involves some
logic change: e.g.  instead of defining `MG_ENABLE_MQTT`, one should
define `MIOT_ENABLE_MQTT`, which should imply `MG_ENABLE_MQTT` as well.

Macro refactoring is going to be done in another PR.

PUBLISHED_FROM=f8b66d5772846126b94223a729a6bd87897f8de6

examples/ESP8266_RTOS/user/user_main.c

@@ -13,7 +13,7 @@
 #define MG_LISTEN_ADDR "80"
 
 #define MG_TASK_STACK_SIZE 4096
-#define MG_TASK_PRIORITY 1
+#define MIOT_TASK_PRIORITY 1
 
 void uart_div_modify(int uart_no, unsigned int freq);
 
@@ -105,5 +105,5 @@ void user_init(void) {
   uart_div_modify(0, UART_CLK_FREQ / 115200);
 
   xTaskCreate(mg_task, (const signed char *) "mongoose", MG_TASK_STACK_SIZE,
-              NULL, MG_TASK_PRIORITY, &s_mg_task_handle);
+              NULL, MIOT_TASK_PRIORITY, &s_mg_task_handle);
 }