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/P2020RDB-PC_36BIT_SPIFLASH_defconfig b/configs/P2020RDB-PC_36BIT_SPIFLASH_defconfig
index f4e3c6e..db594aa 100644
--- a/configs/P2020RDB-PC_36BIT_SPIFLASH_defconfig
+++ b/configs/P2020RDB-PC_36BIT_SPIFLASH_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="P2020RDB,SPIFLASH"
 CONFIG_BOOTDELAY=10
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF=y