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/Itead_Ibox_A20_defconfig b/configs/Itead_Ibox_A20_defconfig
index cb6dfe4..9adebf5 100644
--- a/configs/Itead_Ibox_A20_defconfig
+++ b/configs/Itead_Ibox_A20_defconfig
@@ -5,8 +5,8 @@
 CONFIG_MMC0_CD_PIN="PH1"
 CONFIG_DEFAULT_DEVICE_TREE="sun7i-a20-itead-ibox"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
-CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="SUNXI_GMAC,AHCI,SATAPWR=SUNXI_GPB(8)"
+CONFIG_SPL=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_FPGA is not set