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/imxrt1050-evk_defconfig b/configs/imxrt1050-evk_defconfig
index 0139e73..3f7b670 100644
--- a/configs/imxrt1050-evk_defconfig
+++ b/configs/imxrt1050-evk_defconfig
@@ -68,6 +68,8 @@
 CONFIG_DM_VIDEO=y
 CONFIG_BACKLIGHT_GPIO=y
 CONFIG_SYS_WHITE_ON_BLACK=y
+CONFIG_SPLASH_SCREEN=y
+CONFIG_SPLASH_SCREEN_ALIGN=y
 CONFIG_SHA1=y
 CONFIG_SHA256=y
 CONFIG_HEXDUMP=y