package_release: Add a way to copy the whole directory

Mediatek Genio devices require the device tree overlays stored in devicetree folder to be accessible for genio-flash tool to be able to list them on command line. This MR extends the functionality of the package_release script to treat items ending with "/" as whole directories.

Required for seco-ne/yocto/layers/meta-seconorth-mtk!56 (merged)

Tested with https://git.seco.com/seco-ne/yocto/manifest/-/pipelines/117242