Implement support for the internal flash peripheral

[andysworkshop]

This new feature will permit write access to the internal flash memory so that it can be used as a storage device for application settings.

General support for program/erase operations will be provided as well as higher level classes for easy storage and retrieval of application settings structures. A wear-levelling technique ensures that the 10K erase/program cycle limit of each page is not likely to be reached.

[andysworkshop]

Settings storage/retrieval tested and working on the 64Kb F051 (the STM32F0 discovery board). Commits for this feature are on the internal-flash branch.

There's a new demo flash-internal-settings that shows how to store/retrieve settings structures.

[andysworkshop]

This is now on master