Flash writer proposal

I added a header for a fash_writer, to discuss the API.
I made an [implementation](https://github.com/nuft/nucleo-f302-basic-example/tree/flash-writer-test) for the STM32F3.
We should also discuss how much the master is aware about the flash layout of the MCU. 
(all F3's have 2K pages, other controllers, like the F4, have pages of different sizes.)