Convert CONFIG_SPLASH_SCREEN et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPLASH_SCREEN
   CONFIG_SPLASH_SCREEN_ALIGN
   CONFIG_SPLASHIMAGE_GUARD
   CONFIG_SPLASH_SOURCE

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/mx51evk_defconfig b/configs/mx51evk_defconfig
index f626aca..343816f 100644
--- a/configs/mx51evk_defconfig
+++ b/configs/mx51evk_defconfig
@@ -38,4 +38,5 @@
 CONFIG_USB_ETHER_ASIX=y
 CONFIG_USB_ETHER_SMSC95XX=y
 CONFIG_SYS_WHITE_ON_BLACK=y
+CONFIG_SPLASH_SCREEN=y
 CONFIG_OF_LIBFDT=y