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/include/env_callback.h b/include/env_callback.h
index c583120..62428d1 100644
--- a/include/env_callback.h
+++ b/include/env_callback.h
@@ -41,6 +41,12 @@
 #define SILENT_CALLBACK
 #endif
 
+#ifdef CONFIG_SPLASHIMAGE_GUARD
+#define SPLASHIMAGE_CALLBACK "splashimage:splashimage,"
+#else
+#define SPLASHIMAGE_CALLBACK
+#endif
+
 /*
  * This list of callback bindings is static, but may be overridden by defining
  * a new association in the ".callbacks" environment variable.
@@ -51,6 +57,7 @@
 	"bootfile:bootfile," \
 	"loadaddr:loadaddr," \
 	SILENT_CALLBACK \
+	SPLASHIMAGE_CALLBACK \
 	"stdin:console,stdout:console,stderr:console," \
 	CONFIG_ENV_CALLBACK_LIST_STATIC