Don't consume stderr in merge-squashfs

Part of nerves-project/nerves#876

scripts/merge-squashfs

@@ -184,7 +184,7 @@ find_to_pseudofile "$overlay_dir" >>pseudofile.in
 # remove repeated entries to avoid warnings from mksquashfs
 awk '!x[$1]++' pseudofile.in > pseudofile
 
-unsquashfs "$input_squashfs" >/dev/null 2>/dev/null
+unsquashfs "$input_squashfs" >/dev/null
 cp -Rf "$overlay_dir/." "$workdir/squashfs-root"
 mksquashfs squashfs-root "$output_squashfs" -pf pseudofile -sort "$squashfs_priorities" -noappend -no-recovery -no-progress $NERVES_MKSQUASHFS_FLAGS