lcd: implement a callback for splashimage

On some architectures certain values of splashimage will lead to
a data abort exception.

Document the problem, and implement a callback for splashimage to
reject such values.

Cc: Anatolij Gustschin <agust@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
diff --git a/README b/README
index b8ed1f1..398d70d 100644
--- a/README
+++ b/README
@@ -1530,6 +1530,17 @@
 		allows for a "silent" boot where a splash screen is
 		loaded very quickly after power-on.
 
+		CONFIG_SPLASHIMAGE_GUARD
+
+		If this option is set, then U-Boot will prevent the environment
+		variable "splashimage" from being set to a problematic address
+		(see README.displaying-bmps and README.arm-unaligned-accesses).
+		This option is useful for targets where, due to alignment
+		restrictions, an improperly aligned BMP image will cause a data
+		abort. If you think you will not have problems with unaligned
+		accesses (for example because your toolchain prevents them)
+		there is no need to set this option.
+
 		CONFIG_SPLASH_SCREEN_ALIGN
 
 		If this option is set the splash image can be freely positioned