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/include/configs/zmx25.h b/include/configs/zmx25.h
index 0d5ee14..9a7a27a 100644
--- a/include/configs/zmx25.h
+++ b/include/configs/zmx25.h
@@ -46,7 +46,7 @@
 #define CONFIG_CMDLINE_TAG		/* enable passing of ATAGs	*/
 #define CONFIG_SETUP_MEMORY_TAGS
 #define CONFIG_INITRD_TAG
-#define BOARD_LATE_INIT
+#define CONFIG_BOARD_LATE_INIT
 
 /*
  * Compressions