Remove double square bracket. [[ is a bash or ksh built-in, and cannot be used in a #!/bin/sh script.

Signed-off-by: Robert Langhammer <rlanghammer@web.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>