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/mx31pdk_defconfig b/configs/mx31pdk_defconfig
index 901f992..6299364 100644
--- a/configs/mx31pdk_defconfig
+++ b/configs/mx31pdk_defconfig
@@ -1,7 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_MX31PDK=y
-CONFIG_SPL=y
 CONFIG_BOOTDELAY=1
+CONFIG_SPL=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPI=y