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/ls2080ardb_nand_defconfig b/configs/ls2080ardb_nand_defconfig
index a362f0d..9eec935 100644
--- a/configs/ls2080ardb_nand_defconfig
+++ b/configs/ls2080ardb_nand_defconfig
@@ -1,12 +1,12 @@
 CONFIG_ARM=y
 CONFIG_TARGET_LS2080ARDB=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="SYS_FSL_DDR4, NAND, LS2080A"
 CONFIG_BOOTDELAY=10
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_GREPENV=y
 CONFIG_CMD_MMC=y