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/BSC9131RDB_NAND_defconfig b/configs/BSC9131RDB_NAND_defconfig
index c05ad2a..3bade0c 100644
--- a/configs/BSC9131RDB_NAND_defconfig
+++ b/configs/BSC9131RDB_NAND_defconfig
@@ -1,13 +1,13 @@
 CONFIG_PPC=y
 CONFIG_MPC85xx=y
 CONFIG_TARGET_BSC9131RDB=y
-CONFIG_SPL=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="BSC9131RDB,NAND"
 CONFIG_BOOTDELAY=10
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set