Kconfig: Move SPL settings into their own file

Move the SPL settings into common/spl where most of the SPL code is kept.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/mx23evk_defconfig b/configs/mx23evk_defconfig
index 4d3a4eb..7f0d8f0 100644
--- a/configs/mx23evk_defconfig
+++ b/configs/mx23evk_defconfig
@@ -1,8 +1,8 @@
 CONFIG_ARM=y
 CONFIG_TARGET_MX23EVK=y
-CONFIG_SPL=y
 CONFIG_BOOTDELAY=1
 CONFIG_VERSION_VARIABLE=y
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set