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/ds414_defconfig b/configs/ds414_defconfig
index f95a9df..1003bf3 100644
--- a/configs/ds414_defconfig
+++ b/configs/ds414_defconfig
@@ -2,8 +2,8 @@
 CONFIG_ARCH_MVEBU=y
 CONFIG_TARGET_DS414=y
 CONFIG_DEFAULT_DEVICE_TREE="armada-xp-synology-ds414"
-CONFIG_SPL=y
 CONFIG_BOOTDELAY=3
+CONFIG_SPL=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_SF=y