package/skeleton: split out into skeleton-custom

For the custom skeleton, we practicaly do nothing, except ensure it
contains the basic, required directories, and that those are properly
setup wrt. merged /usr.

Furthermore, our current skeleton is not fit for systemd, and we'll
have to split things out into various skeletons.

So, off-load the custom skeleton into its own package.

Thus, the existing skeleton package is now limited to:

  - when using our default skeleton, install and tweak it properly;

  - when using a custom skeleton, do nothing except for depending on
    the skeleton-custom package.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Arnout: split off in a separate patch doing only this]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
diff --git a/package/Config.in b/package/Config.in
index 86d714d..f409f5a 100644
--- a/package/Config.in
+++ b/package/Config.in
@@ -2,6 +2,7 @@
 
 	source "package/busybox/Config.in"
 	source "package/skeleton/Config.in"
+	source "package/skeleton-custom/Config.in"
 
 menu "Audio and video applications"
 	source "package/alsa-utils/Config.in"