package/skeleton: add macro to rsync skeleton directory

package/skeleton: add macro to rsync skeleton directory

The rsync command is currently called from a single location. In
follow-up patches, it will be called from various places then.

It will be easier to update and fix this command if it is defined in a
single location, rather than if duplicated in many places.

Define a macro that skeleton packages can call to install their skeleton
directories. Since at least one occurrence of it will want to install in
staging, the macro must accept a destination path, rather than hard-code
target as the destination.

Since it is to be used by other skeleton packages, define it early,
outside of any conditional block.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>