Make all methods in `NVMController` pure virtual.

[microbit-carlos]

This saves about 184 bytes of flash with gcc v10.3.1 & the codal-microbit-v2 build configuration.

And, as it might be removing the entry in the vtable for the derived classes, it could be a bit more efficient too.

Related to:

• https://github.com/lancaster-university/codal-core/pull/162
• https://github.com/lancaster-university/codal-core/pull/163