esp8266/ets_alt_task: Make FIRST_PRIO=0 to cover all task priorities.

FIRST_PRIO=2 works but using 0 is hopefully safer, and can anyway be
optimised in the future.

esp8266/ets_alt_task.c

@@ -24,7 +24,7 @@ static void (*idle_cb)(void *);
 static void *idle_arg;
 
 #if ESP_SDK_VERSION >= 010500
-# define FIRST_PRIO 3
+# define FIRST_PRIO 0
 #else
 # define FIRST_PRIO 0x14
 #endif