* Make ExtUI respect MAXTEMP limits
- Temperatures are now clamped by MAXTEMP limits rather than arbitrary values.
* Speed up USB init, add status
- Speed up USB initialization
- Show status message if init failed
* Enable status messages for EXTENSIBLE_UI
* Adjust max limit to MAX_TEMP - 15
* Misc. tweaks to formatting, const, etc.
- Remove `print_job_timer.stop()` from `disable_all_heaters`
- Call `print_job_timer.stop()` for relevant `disable_all_heaters()`.
- Split up `kill()` for watchdog interrupt safety
- Fix an error in the return value of `sd_mmc_spi_read_capacity` which was causing the host OS to read beyond the last sector in the card.
- Clean up the USB flashdrive code and add better debugging.
Scott Lahteine