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/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index 808ac34..eb1ea38 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -3,7 +3,6 @@
 CONFIG_PCI=y
 CONFIG_DEFAULT_DEVICE_TREE="sandbox"
 CONFIG_I8042_KEYB=y
-CONFIG_SPL=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_FIT_SIGNATURE=y
@@ -17,6 +16,7 @@
 CONFIG_BOOTSTAGE_STASH_SIZE=0x4096
 CONFIG_CONSOLE_RECORD=y
 CONFIG_CONSOLE_RECORD_OUT_SIZE=0x1000
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_CPU=y
 CONFIG_CMD_LICENSE=y