Report bed standby temp in M408 json responses

[gtj0]

Unless I'm missing it, you can set the bed standby temp with M140 Rnnn but there's no way to see what it currently is in any of the json responses.

[gtj0]

Just created a pull request for this...
https://github.com/dc42/RepRapFirmware/pull/248

gtjoseph opened this pull request in dc42/RepRapFirmware

closed Add standby temp to JSON bed temps #248