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.
Can now choose at boot up whether the USB device is CDC+MSC or CDC+HID.
Choice is made by an option in boot.py, with default being CDC+MSC.
HID+MSC is not currently supported, but should be easy to implement.
Boot up now has ability to change the reset mode: hold down USR switch
while booting and LEDs will count from 1 to 7 to indicate the boot mode.
Release USR when correct mode is selected. Current modes are 1 (normal
boot), 2 (safe mode), 3 (reset FS mode).
New USB HAL is quite a bit improved over previous one. Now has better
callbacks and flow control.
REPL over USB CDC now works as before, except for soft-reset (since USB
driver uses malloc...).
