Create 'custom' subdirectory.

    This directory is a place for users to add custom packages
    and patches outside of the rest of the openpower package
    tree.  This allows them to more easily maintain a fork of
    op-build and enhance it with their own packages.
diff --git a/openpower/Config.in b/openpower/Config.in
index 19007fe..19a2859 100644
--- a/openpower/Config.in
+++ b/openpower/Config.in
@@ -8,4 +8,8 @@
 endif
 endmenu
 
+menu "Custom Packages"
+source "$BR2_EXTERNAL/custom/Config.in"
+endmenu
+
 endmenu