usbdevice/core: add reset function to support hotplug

When the USB got RESET packet from the host and the address is setup,
all the classes will got reset. The reset is done by class stop and than
class run. So the classes should reset their internal state in
class_{run,stop}.

Besides, the USB device driver could post a USB_MSG_RESET message on
every RESET packet.

cdc_vcom: reset the vcom state in proper cases

Reset the vcom state in class_{run,stop} and when the PC is not
receiving data from me.

For pull request

simulator: fix serial_write

simulator: fix serial_write

For pull request

add missing dfs_mkfs prototype in dfs_fs.h

Pulls

Static usbd

printf("%s") will print a NULL terminated string, while rt_serial_write
should print buffer for size length. This also fix segfault when the
string is not null terminated but specified by size.

Static usbd

Although currently the only message send to the queue is struct
udev_msg, in order to keep backward compacity and for extention in the
future, we set the size of message to 32.

It also add two configurations for the USB device even loop thread:

    RT_USBD_THREAD_STACK_SZ to set the stack size, default to 2048
    RT_USBD_THREAD_PRIO     to set the priority, default to 8

You can overwrite the default values in rtconfig.h

serial: use RT_SERIAL_RB_BUFSZ to define the size of ring buffer.

Fix usb serial

We start the sending transaction in SOF handler. But if the data is
already sending, start an other transaction will cause data lose.
Implement a state machine is cdc_vcom and avoid that.

Sort the switch cases by the frequency of the events.

The data filled into dcd_ep_write does not to be limited by
MaxPacketSize.

This commit set the buffer for packet to CDC_MaxPacketSize which is a
reasonable value for it. However, maybe we should make
CDC_{RX,TX}_BUFSIZE configurable as well.

Ringbuffer

The new implementation use the mirroring method to distinguish full
buffer and the empty buffer. It also support buffers of arbitrary size.