stm32: Factor out flash and SPI block-device code to separate files.

Prior to this patch, storage.c was a combination of code that handled
either internal flash or external SPI flash and exposed one of them as a
block device for the local storage.  It was also exposed to the USB MSC.

This patch splits out the flash and SPI code to separate files, which each
provide a general block-device interface (at the C level).  Then storage.c
just picks one of them to use as the local storage medium.  The aim of this
factoring is to allow to add new block devices in the future and allow for
easier configurability.