[BUG] Ultimaker 2+ shall have BLOCK_BUFFER_SIZE of 8 instead of 16 when SD is enabled only

### Did you test the latest `bugfix-2.1.x` code?

Yes, and the problem still exists.

### Bug Description

Marlin will exit abnormally, with no error message and restart when printing from SD card.

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### Additional information

https://community.ultimaker.com/topic/44413-marlin-2121-on-um2-stops-and-reboots-when-starting-print/

https://github.com/MarlinFirmware/Configurations/blob/release-2.1.2.5/config/examples/Ultimaker/Ultimaker%202%2B/Configuration_adv.h

Open the linked Ultimaker 2+ Configuration_adv.h and inspect the existing BLOCK_BUFFER_SIZE setting and its SD-related condition. Confirm that the Ultimaker 2+ configuration uses a value of 8 when SD is enabled only, then verify the configuration matches the reported printing case.

Written by the indexing model from the issue text.