base-files: fix option to make /var persistent

The option was initially named TARGET_ROOTFS_LN_VAR_TMP, and the check
was correct. When renaming the option to something more suitable, the
check was changed to check for n, but when an option is not set, it's
not n but empty. This results in the check always evaluating to false.
Fix the check by checking for y with ifneq.

Fixes: 57807f50de ("base-files: add option to make /var persistent")

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

package/base-files/Makefile

@@ -181,7 +181,7 @@ define Package/base-files/install
 	mkdir -p $(1)/www
 	mkdir -p $(1)/root
 	$(LN) /proc/mounts $(1)/etc/mtab
-ifeq ($(CONFIG_TARGET_ROOTFS_PERSIST_VAR),n)
+ifneq ($(CONFIG_TARGET_ROOTFS_PERSIST_VAR),y)
 	rm -f $(1)/var
 	$(LN) tmp $(1)/var
 else