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/common/Kconfig b/common/Kconfig
index 63c4a67..35f4279 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -213,3 +213,5 @@
 	  version as printed by the "version" command.
 	  Any change to this variable will be reverted at the
 	  next reset.
+
+source "common/spl/Kconfig"