POST: allow redefinition of post_word_load/store

The predefinde post_word_load/store functions do not fit all boards,
so we introduce a way to define post_word_load/store as externs in
post.h that then can be defined in board specific files. This is done
with the CONFIG_POST_EXTERNAL_WORD_FUNCS #define

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>

include/post.h

@@ -33,6 +33,7 @@
 
 #if defined(CONFIG_POST) || defined(CONFIG_LOGBUFFER)
 
+#ifndef CONFIG_POST_EXTERNAL_WORD_FUNCS
 #ifdef CONFIG_SYS_POST_WORD_ADDR
 #define _POST_WORD_ADDR	CONFIG_SYS_POST_WORD_ADDR
 #else
@@ -85,6 +86,13 @@ static inline void post_word_store (ulong value)
 {
 	out_le32((volatile void *)(_POST_WORD_ADDR), value);
 }
+
+#else
+
+extern ulong post_word_load(void);
+extern void post_word_store(ulong value);
+
+#endif /* CONFIG_POST_EXTERNAL_WORD_FUNCS */
 #endif /* defined (CONFIG_POST) || defined(CONFIG_LOGBUFFER) */
 #endif /* __ASSEMBLY__ */