[KConfig] Change the RT_DFS_ELM_USE_LFN cfg to choice.

components/dfs/KConfig

@@ -29,15 +29,29 @@ if RT_USING_DFS
             int "OEM code page"
             default 437
         
+        choice 
+            prompt "Support long file name"
+            default RT_DFS_ELM_USE_LFN_0
+
+            config RT_DFS_ELM_USE_LFN_0
+                bool "0: LFN disable"
+
+            config RT_DFS_ELM_USE_LFN_1
+                bool "1: LFN with static LFN working buffer"
+
+            config RT_DFS_ELM_USE_LFN_2
+                bool "2: LFN with dynamic LFN working buffer on the stack"
+            
+            config RT_DFS_ELM_USE_LFN_3
+                bool "3: LFN with dynamic LFN working buffer on the heap"
+        endchoice
+        
         config RT_DFS_ELM_USE_LFN
-            int "Support long file name"
-            default 0
-            range 0 3
-            help
-                0: LFN disable
-                1: LFN with static LFN working buffer
-                2: LFN with dynamic LFN working buffer on the stack
-                3: LFN with dynamic LFN working buffer on the heap
+            int
+            default 0 if RT_DFS_ELM_USE_LFN_0
+            default 1 if RT_DFS_ELM_USE_LFN_1
+            default 2 if RT_DFS_ELM_USE_LFN_2
+            default 3 if RT_DFS_ELM_USE_LFN_3
         
         config RT_DFS_ELM_MAX_LFN
             int "Maximal size of file name length"