systemd.bbclass: Check for existence of the symlink too

systemd.bbclass: Check for existence of the symlink too

If the packages' service file is a link to parameterized service file
provided by different package [1] then link is dangling link at
do_package stage and os.path.exists(path) returns False even if the link
exists. Replace os.path.exists with lexists to fix this issue.

[1]
An example:
  Package A provides myservice@.service

  Package B depends and rdepends on A and provides:
  myservice@B.service -> myservice@.service

Signed-off-by: Pavel Zhukov <pavel@zhukoff.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>