Using SysTick to do the counting and dispatch of the flash storage idle
handler is more efficient than requiring a dedicated hardware timer.
No new counter is needed, just the existing uwTick variable. The
processing is not actually done in the SysTick IRQ, it is deferred to
the flash IRQ (which runs at lower priority).
Blanket wide to all .c and .h files. Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.
Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.
Internal flash used for the filesystem is now written (from the cache)
only after a 5s delay, or when a file is closed, or when the drive is
unmounted from the host. This delay means that multiple writes can
accumulate in the cache, and leads to less writes to the flash, making
it last longer.
It's implemented by a high-priority interrupt that takes care of flash
erase and write, and flushing the cache.
This is still only an interim solution for the flash filesystem. It
eventually needs to be replaced with something that uses less RAM for
the cache, something that can use more of the flash, and something that
does proper wear levelling.
