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/pico-imx6ul_defconfig b/configs/pico-imx6ul_defconfig
index 5c841c3..da51e37 100644
--- a/configs/pico-imx6ul_defconfig
+++ b/configs/pico-imx6ul_defconfig
@@ -74,3 +74,5 @@
 CONFIG_USB_GADGET_PRODUCT_NUM=0xa4a5
 CONFIG_CI_UDC=y
 CONFIG_VIDEO=y
+CONFIG_SPLASH_SCREEN=y
+CONFIG_SPLASH_SCREEN_ALIGN=y