tzdata: We shouldn't override the localtime if it is valid

tzdata: We shouldn't override the localtime if it is valid

The code where mistakenly replacing the localtime file setting so we
end with a copy of file instead of a symbolic link. This fixes it so
now, we'll only do that in case the link is pointing to invalid data.

Change-Id: I16dfa5ea4f293c48bb396f4e23a2ea53e6c9e745
(From OE-Core rev: a9b42c8e85332a65788b1434f926186d4887b287)

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>