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-imx6_defconfig b/configs/pico-imx6_defconfig
index 2316544..33adef8 100644
--- a/configs/pico-imx6_defconfig
+++ b/configs/pico-imx6_defconfig
@@ -88,3 +88,5 @@
 CONFIG_SYS_WHITE_ON_BLACK=y
 # CONFIG_PANEL is not set
 CONFIG_VIDEO_IPUV3=y
+CONFIG_SPLASH_SCREEN=y
+CONFIG_SPLASH_SCREEN_ALIGN=y