base-files: fix option to make /var persistent

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: 57807f50ded6 ("base-files: add option to make /var persistent")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>