layer.conf: fix syntax error in PATH setting

Commit 05a87be5 introduced
a Python conditional expression when updating PATH that
generates syntax warnings in bitbake-cookerdaemon.log:

  Var <PATH[:=]>:1: SyntaxWarning: "is not" with a literal. Did you mean "!="?

Fix this by using the more appropriate '!=' comparison
operator.

Signed-off-by: Matt Madison <matt@madison.systems>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2e753a12)
Signed-off-by: Steve Sakoman <steve@sakoman.com>