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/zynq_zybo_defconfig b/configs/zynq_zybo_defconfig
index 4b85cdb..e5ecb79 100644
--- a/configs/zynq_zybo_defconfig
+++ b/configs/zynq_zybo_defconfig
@@ -2,11 +2,11 @@
 CONFIG_SYS_CONFIG_NAME="zynq_zybo"
 CONFIG_ARCH_ZYNQ=y
 CONFIG_DEFAULT_DEVICE_TREE="zynq-zybo"
-CONFIG_SPL=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_SYS_NO_FLASH=y
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Zynq> "
 # CONFIG_CMD_IMLS is not set