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/gwventana_defconfig b/configs/gwventana_defconfig
index 7cf5cf5..6d7856d 100644
--- a/configs/gwventana_defconfig
+++ b/configs/gwventana_defconfig
@@ -3,13 +3,13 @@
 CONFIG_TARGET_GW_VENTANA=y
 CONFIG_SPL_STACK_R_ADDR=0x18000000
 CONFIG_DM_SERIAL=y
-CONFIG_SPL=y
-CONFIG_SPL_STACK_R=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6QDL"
 CONFIG_BOOTDELAY=3
+CONFIG_SPL=y
+CONFIG_SPL_STACK_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Ventana > "
 CONFIG_CMD_BOOTZ=y