lxc-unprivileged: install /etc/sub{uid,gid}

Create default /etc/sub{uid,gid} files to use for unprivileged
containers.

Appearently these file are not generated/owned by any other OpenWRT
package or buildroot configuration setting.
If such a setting or package exists this commit should probably be
reverted an modified in an extra dependency for that setting/package.

In these default the range 100000-165535 is assigned to root
sub{uid,gid}.

utils/lxc/Makefile

@@ -100,6 +100,8 @@ define Package/lxc-unprivileged/install
 	$(INSTALL_DIR) $(1)/etc/uci-defaults $(1)/etc/init.d
 	$(INSTALL_DATA) ./files/lxc-unprivileged.defaults $(1)/etc/uci-defaults/lxc-unprivileged
 	$(INSTALL_BIN) ./files/lxc-unprivileged.init $(1)/etc/init.d
+	$(INSTALL_DATA) ./files/subuid $(1)/etc/subuid
+	$(INSTALL_DATA) ./files/subgid $(1)/etc/subgid
 endef
 
 define Package/lxc/config

utils/lxc/files/subgid

@@ -0,0 +1 @@
+root:100000:65536

utils/lxc/files/subuid

@@ -0,0 +1 @@
+root:100000:65536