cleaned up temperature string generation

replaced all kinds of versions with
itoa_fixedpoint()

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>
changed strcpy_P() to strncpy_P()

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>
replaced ECMD_OUTPUTBUF_LENGTH with len

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>
Merge pull request #259 from mguentner/onewire-get-fix

cleaned up temperature string generation