trees: fix listfile check in copy_elf()

don't check that the variable is empty

check that the file itself exists or not

this should fix the recent build issues

Signed-off-by: Leah Rowe <leah@libreboot.org>

script/trees

@@ -288,7 +288,7 @@ mkpayload_grub()
 
 copy_elf()
 {
-	[ -z "$listfile" ] && return 0
+	[ -f "$listfile" ] || return 0
 
 	while read -r f; do
 		[ -f "$cdir/$f" ] && x_ cp "$cdir/$f" "$dest_dir"