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>