Use stdioERRNO_THREAD_LOCAL_OFFSET macro for ERRNO setter/getter (#82)

Use stdioERRNO_THREAD_LOCAL_OFFSET macro for ERRNO setter/getter

Description
-----------

The previously used ffconfigCWD_THREAD_LOCAL_INDEX has the same value
but the stdioERRNO_THREAD_LOCAL_OFFSET macro is more concise.

The compilation will be unchanged as the preprocessor will generate the
same output as before.

Test Steps
-----------

No need to test. This is just for more clarity in the codebase and
navigation.

Checklist:
----------
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- [ ] I have tested my changes. No regression in existing tests.
- [ ] I have modified and/or added unit-tests to cover the code changes
in this Pull Request.

Related Issue
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Author: StefanBalt

include/ff_stdio.h

@@ -136,14 +136,14 @@
         /* Cast from a numeric value to a pointer. */
         void * pvValue = ( void * ) ( xErrno );
 
-        vTaskSetThreadLocalStoragePointer( NULL, ffconfigCWD_THREAD_LOCAL_INDEX, pvValue );
+        vTaskSetThreadLocalStoragePointer( NULL, stdioERRNO_THREAD_LOCAL_OFFSET, pvValue );
     }
 
     static portINLINE int stdioGET_ERRNO( void )
     {
         void * pvResult;
 
-        pvResult = pvTaskGetThreadLocalStoragePointer( ( TaskHandle_t ) NULL, ffconfigCWD_THREAD_LOCAL_INDEX );
+        pvResult = pvTaskGetThreadLocalStoragePointer( ( TaskHandle_t ) NULL, stdioERRNO_THREAD_LOCAL_OFFSET );
         /* Cast from a pointer to a number of the same size. */
         intptr_t xErrno = ( intptr_t ) pvResult;
         /* Cast it to an integer. */