add littfs demo on tos_evb_aiot board

board/TencentOS_tiny_EVB_AIoT/littlefs/main.c

@@ -0,0 +1,28 @@
+#include "mcu_init.h"
+
+#define APPLICATION_TASK_STK_SIZE       4096
+k_task_t application_task;
+uint8_t application_task_stk[APPLICATION_TASK_STK_SIZE];
+
+extern void application_entry(void *arg);
+
+#pragma weak application_entry
+void application_entry(void *arg)
+{
+    while (1) {
+        printf("This is a demo task,please use your task entry!\r\n");
+        tos_task_delay(1000);
+    }
+}
+
+int main(void)
+{
+    board_init();
+    PRINTF("Welcome to TencentOS tiny(%s)\r\n", TOS_VERSION);
+    tos_knl_init(); // TencentOS Tiny kernel initialize
+    tos_task_create(&application_task, "application_task", application_entry, NULL, 4, application_task_stk, APPLICATION_TASK_STK_SIZE, 0);
+    tos_knl_start();
+    
+    while (1);
+}
+