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/mcx_defconfig b/configs/mcx_defconfig
index feea816..c6ac8d4 100644
--- a/configs/mcx_defconfig
+++ b/configs/mcx_defconfig
@@ -1,9 +1,9 @@
 CONFIG_ARM=y
 CONFIG_OMAP34XX=y
 CONFIG_TARGET_MCX=y
-CONFIG_SPL=y
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="mcx # "
 # CONFIG_CMD_IMI is not set