vim: set PACKAGECONFIG idiomatically

Don't set an empty default value and them immediately assign to it.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit d7565241)
Signed-off-by: Steve Sakoman <steve@sakoman.com>