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/mx1ads.h b/include/configs/mx1ads.h
index 0643312..9c1b255 100644
--- a/include/configs/mx1ads.h
+++ b/include/configs/mx1ads.h
@@ -45,7 +45,7 @@
 /* #define _CONFIG_UART2 */		/* internal uart 2 */
 /* #define CONFIG_SILENT_CONSOLE */	/* use this to disable output */
 
-#define BOARD_LATE_INIT		1
+#define CONFIG_BOARD_LATE_INIT
 #define USE_920T_MMU		1
 
 #if 0