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/woodburn_sd_defconfig b/configs/woodburn_sd_defconfig
index 3b38130..5dea03e 100644
--- a/configs/woodburn_sd_defconfig
+++ b/configs/woodburn_sd_defconfig
@@ -1,8 +1,8 @@
 CONFIG_ARM=y
 CONFIG_TARGET_WOODBURN_SD=y
-CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/woodburn/imximage.cfg"
 CONFIG_BOOTDELAY=3
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="woodburn U-Boot > "
 CONFIG_CMD_MMC=y