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/tx25.h b/include/configs/tx25.h
index 6380bb8..8cb57ff 100644
--- a/include/configs/tx25.h
+++ b/include/configs/tx25.h
@@ -137,7 +137,7 @@
 #define CONFIG_FEC_MXC_PHYADDR		0x1f
 #define CONFIG_MII
 #define CONFIG_CMD_NET
-#define BOARD_LATE_INIT
+#define CONFIG_BOARD_LATE_INIT
 #define CONFIG_ENV_OVERWRITE
 
 #define CONFIG_BOOTDELAY	5