subsys: usb: rework USB DFU class driver

This patch moves USB DFU class driver to subsys/usb/class.

For the first the USB DFU class driver depends on DFU image
manager and partition layout and is limited to use as an
application for the bootloader. The driver fetches the
information about the flash, erase block size, write block
size and partitions offset from the DT now. The driver has
two interfaces associated with the two partitions "SLOT-0"
and "SLOT-1". The "SLOT-0" can only be read.

In the following work the class driver can be extended so
that it can be used from the bootloader and update a flash
region directly from the bootloader.

Signed-off-by: Johann Fischer <j.fischer@phytec.de>