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/mx53evk.h b/include/configs/mx53evk.h
index 2033492..034fa87 100644
--- a/include/configs/mx53evk.h
+++ b/include/configs/mx53evk.h
@@ -44,7 +44,7 @@
 #define CONFIG_SYS_MALLOC_LEN		(CONFIG_ENV_SIZE + 2 * 1024 * 1024)
 
 #define CONFIG_BOARD_EARLY_INIT_F
-#define BOARD_LATE_INIT
+#define CONFIG_BOARD_LATE_INIT
 #define CONFIG_MXC_GPIO
 
 #define CONFIG_MXC_UART