Make motion sensors `configure()` & `requestUpdate()` pure virtual.

[microbit-carlos]

This saves about 152 bytes of flash with gcc v10.3.1 & the codal-microbit-v2 build configuration.

And, as it would be removing the entries in the vtable for the derived classes, it should be a bit more efficient too.

Related to:

• https://github.com/lancaster-university/codal-core/pull/163
• https://github.com/lancaster-university/codal-core/pull/164