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/[email protected]/

Fixes: 1eaad3a ("tools/bootconfig: Use per-group/all enable option in ftrace2bconf script")
Signed-off-by: Antonio Alvarez Feijoo <[email protected]>
Cc: [email protected]
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>

Author: aafeijoo-suse

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