common:splash: use __weak

This not only looks a bit better it also prevents a
warning with W=1 (no previous prototype).

cc: agust@denx.de
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>

common/splash.c

@@ -23,15 +23,11 @@
 #include <common.h>
 #include <splash.h>
 
-int __splash_screen_prepare(void)
+__weak int splash_screen_prepare(void)
 {
 	return 0;
 }
 
-int splash_screen_prepare(void)
-	__attribute__ ((weak, alias("__splash_screen_prepare")));
-
-
 #ifdef CONFIG_SPLASH_SCREEN_ALIGN
 void splash_get_pos(int *x, int *y)
 {