scripts/config: fix variable substitution command

Commit 229455bc02b87f7128f190c4491b4ceffff38648 accidentally changed the
separator between sed `s' command and its parameters from ':' to '/'.

Revert this change.
Reported-and-tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Clement Chauplannaz <chauplac@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/config

@@ -82,7 +82,7 @@ txt_subst() {
 	local infile="$3"
 	local tmpfile="$infile.swp"
 
-	sed -e "s/$before/$after/" "$infile" >"$tmpfile"
+	sed -e "s:$before:$after:" "$infile" >"$tmpfile"
 	# replace original file with the edited one
 	mv "$tmpfile" "$infile"
 }