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/mx28evk_nand_defconfig b/configs/mx28evk_nand_defconfig
index 26c9762..91ad0f0 100644
--- a/configs/mx28evk_nand_defconfig
+++ b/configs/mx28evk_nand_defconfig
@@ -1,9 +1,9 @@
 CONFIG_ARM=y
 CONFIG_TARGET_MX28EVK=y
-CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="ENV_IS_IN_NAND"
 CONFIG_BOOTDELAY=1
 CONFIG_VERSION_VARIABLE=y
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set