Kconfig: Move boot menu into common/

Most of the boot options are in common/Kconfig but that file is already
extremely large. Create a new Kconfig.boot to hold the boot options.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Kconfig b/common/Kconfig
index 9c20a97..f234240 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -1,3 +1,5 @@
+source "common/Kconfig.boot"
+
 menu "Boot timing"
 
 config BOOTSTAGE