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/MPC8313ERDB_NAND_66_defconfig b/configs/MPC8313ERDB_NAND_66_defconfig
index be0e274..f9e5439 100644
--- a/configs/MPC8313ERDB_NAND_66_defconfig
+++ b/configs/MPC8313ERDB_NAND_66_defconfig
@@ -1,11 +1,11 @@
 CONFIG_PPC=y
 CONFIG_MPC83xx=y
 CONFIG_TARGET_MPC8313ERDB=y
-CONFIG_SPL=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="SYS_66MHZ,NAND"
 CONFIG_BOOTDELAY=6
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_GPIO=y