Sansa AMS: fix system_exception_wait()

If a panicf() is called while a button is still pressed, the Sansa would
reboot immediately with no chance to see the message

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@23490 a1c6a512-1295-4272-9138-f99709370657

firmware/target/arm/as3525/system-as3525.c

@@ -327,6 +327,9 @@ void system_reboot(void)
 
 void system_exception_wait(void)
 {
+    /* wait until button release (if a button is pressed) */
+    while(button_read_device());
+    /* then wait until next button press */
     while(!button_read_device());
 }