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/pxm2_defconfig b/configs/pxm2_defconfig
index e659055..d66a211 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -104,3 +104,5 @@
 # CONFIG_VIDEO_SW_CURSOR is not set
 CONFIG_SYS_CONSOLE_BG_COL=0xff
 CONFIG_SYS_CONSOLE_FG_COL=0x00
+CONFIG_SPLASH_SCREEN=y
+CONFIG_SPLASH_SCREEN_ALIGN=y