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/brppt1_nand_defconfig b/configs/brppt1_nand_defconfig
index 9c7c07e..1955e33 100644
--- a/configs/brppt1_nand_defconfig
+++ b/configs/brppt1_nand_defconfig
@@ -1,10 +1,10 @@
 CONFIG_ARM=y
 CONFIG_TARGET_BRPPT1=y
-CONFIG_SPL=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1,NAND"
 CONFIG_BOOTDELAY=-2
 CONFIG_VERSION_VARIABLE=y
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set