define RT_USING_BIG_ENDIAN for the long-term version

src/Kconfig

@@ -8,6 +8,10 @@ config RT_NAME_MAX
         Each kernel object, such as thread, timer, semaphore etc, has a name,
         the RT_NAME_MAX is the maximal size of this object name.
 
+config RT_USING_BIG_ENDIAN
+    bool
+    default n
+
 config RT_USING_ARCH_DATA_TYPE
     bool "Use the data types defined in ARCH_CPU"
     default n