package/openssh: Set /var/empty permissions

The openssh privilege separation feature, enabled by default,
requires that the path /var/empty exists and has certain permissions
(not writable by the sshd user). Note that nothing ever gets writting
in this directory, so it works fine on a readonly rootfs.

See README.privsep included as part of the openssh distribution.

Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit f85665c58562a967e3dcb5f72fa8af0a606c9274)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/openssh/openssh.mk

@@ -22,6 +22,10 @@ define OPENSSH_USERS
 	sshd -1 sshd -1 * - - - SSH drop priv user
 endef
 
+define OPENSSH_PERMISSIONS
+	/var/empty d 755 root root - - - - -
+endef
+
 ifeq ($(BR2_TOOLCHAIN_SUPPORTS_PIE),)
 OPENSSH_CONF_OPTS += --without-pie
 endif