cosmetic: s/BOARD_LATE_INIT/CONFIG_BOARD_LATE_INIT

This renames BOARD_LATE_INIT to CONFIG_BOARD_LATE_INIT.
Along the way it removes some leftover

 #define BOARD_LATE_INIT		1

and adds some basic documentation for board specific
callbacks in README.

Signed-off-by: Helmut Raiger <helmut.raiger@hale.at>
Acked-by: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/lib/board.c b/arch/arm/lib/board.c
index bae2892..368fc5d 100644
--- a/arch/arm/lib/board.c
+++ b/arch/arm/lib/board.c
@@ -590,7 +590,7 @@
 	}
 #endif
 
-#ifdef BOARD_LATE_INIT
+#ifdef CONFIG_BOARD_LATE_INIT
 	board_late_init();
 #endif