deploy: Don't copy xattrs for devicetree

deploy: Don't copy xattrs for devicetree

xref: https://github.com/coreos/fedora-coreos-tracker/issues/1808

For the kernel/initramfs that we copy to `/boot`
we use an explicit relabeling today, ignoring the source SELinux
context.

When we added handling for devicetree it reuse the `copy_dir_recurse`
we have for `etc` handling, and that copied the source xattrs.

Let's ensure that the devicetree is also `boot_t` by *not* copying
xattrs and relying on the default labeling.

Signed-off-by: Colin Walters <walters@verbum.org>