update ds18b20.c source code, add break when find ds18b20

Guo Wenxue

2018-10-21 6ad80326dd46f2e557367f7845f91ed1949a07e9

tools/rpi_crosstool.tar.bz2 does not exist in tree 811652f898f1da9988b796ec985a024b6b6697a6