Changes are:
- Set the feature level for each MCU: CORE features for SAMD21, and EXTRA
features for SAMD51.
- Remove all definitions that are included in the core feature level.
- Keep the default settings for feature level and float, to make the choice
obvious.
It can be enabled/disabled by a configuration switch. The code size
increase is 308 bytes, but it requires RAM space for buffers, the larger
UART object and root pointers.
