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/P1021RDB-PC_36BIT_SDCARD_defconfig b/configs/P1021RDB-PC_36BIT_SDCARD_defconfig
index eca4c59..4c86983 100644
--- a/configs/P1021RDB-PC_36BIT_SDCARD_defconfig
+++ b/configs/P1021RDB-PC_36BIT_SDCARD_defconfig
@@ -2,13 +2,13 @@
 CONFIG_MPC85xx=y
 CONFIG_TARGET_P1_P2_RDB_PC=y
 CONFIG_PHYS_64BIT=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="P1021RDB,SDCARD"
 CONFIG_BOOTDELAY=10
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF=y