Add self-test function for flash driver

[KomaGR]

Add a function that will test the flash (and driver) by writing to it and reading back. (Potentially other operations, too.)

Perhaps it would make sense to organize this as a separate build of the firmware that a definition in config.h controls. E.g., WARP_BUILD_ENABLE_SELFTEST. This way the self-test code will not take up important space in production builds.