fablabbcn/smartcitizen-kit-20: Bug fix pre-release

This release includes:

• Light sensor reporting zero fix.
• Light sensor dynamic integration time.
• Improvements on SHT31 driver.
• PM sensor reading failures are reported as null and not published.
• Adjusted max battery voltage to be sure full charge feedback is shown.
• Some other minor fixes.

Please follow the SAM update guide on our documentation.