Add leading / to paths in Contents-firmware indices.

With this change, hw-detect's check-missing-firmware.sh can use grep -f
with a file listing absolute paths to required firmware files.

tools/make_disc_trees.pl

@@ -1243,7 +1243,7 @@ sub add_firmware_stuff {
     while (defined(my $line = <DPKGC>)) {
 	chomp $line;
 	if ($line =~ m,^[-|l]\S+\s+\S+\s+\d+\s+\S+\s+\S+\s+./(\S+/firmware/\S+)$,) {
-	    printf OFILE "%-55s %s\n", $1, $base_file;
+	    printf OFILE "%-55s %s\n", "/$1", $base_file;
 	}
     }
     close DPKGC;