POST/arm: adaptations needed for POST on ARM to work

For post to run on ARM, 3 things are needed:
- post_log_word to be defined in gd
- a post.h include in arch/arm/lib/board.c

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>