tools/bootconfig: fix single & used for logical condition

A single & will create a background process and return true, so the grep
command will run even if the file checked in the first condition does not
exist.

Link: https://lore.kernel.org/all/20230112114215.17103-1-antonio.feijoo@suse.com/

Fixes: 1eaad3ac ("tools/bootconfig: Use per-group/all enable option in ftrace2bconf script")
Signed-off-by: Antonio Alvarez Feijoo <antonio.feijoo@suse.com>
Cc: stable@vger.kernel.org
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

tools/bootconfig/scripts/ftrace2bconf.sh

@@ -93,7 +93,7 @@ referred_vars() {
 }
 
 event_is_enabled() { # enable-file
-	test -f $1 & grep -q "1" $1
+	test -f $1 && grep -q "1" $1
 }
 
 per_event_options() { # event-dir