esp8266/ets_alt_task: Update for vendor SDK 1.5.0.

SDK 1.5.0 has a task with priority 3: ets_task(401001f4, 3, 3fff9808, 4).
Recognizing SDK version requires the latets esp-open-sdk build.

esp8266/ets_alt_task.c

@@ -2,6 +2,7 @@
 #include "osapi.h"
 #include "os_type.h"
 #include "ets_sys.h"
+#include <esp_sdk_ver.h>
 #include "etshal.h"
 #include "user_interface.h"
 
@@ -22,7 +23,11 @@ struct task_entry {
 static void (*idle_cb)(void *);
 static void *idle_arg;
 
+#if ESP_SDK_VERSION >= 010500
+# define FIRST_PRIO 3
+#else
 # define FIRST_PRIO 0x14
+#endif
 #define LAST_PRIO 0x20
 #define PRIO2ID(prio) ((prio) - FIRST_PRIO)