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/sandbox_defconfig b/configs/sandbox_defconfig
index acd7093..41de434 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -247,6 +247,7 @@
 CONFIG_VIDEO_DSI_HOST_SANDBOX=y
 CONFIG_OSD=y
 CONFIG_SANDBOX_OSD=y
+CONFIG_SPLASH_SCREEN_ALIGN=y
 CONFIG_W1=y
 CONFIG_W1_GPIO=y
 CONFIG_W1_EEPROM=y