samples: usb: add sample for the new USB DFU implementation

The sample defines an image that uses RAMdisk and also provides an
option to enable USB DFU flash backend for the "slot-1" flash partition.
Assuming the user runs this sample from the slot-0 partition, uploading
to flash should work fine for evaluation purposes.

If the sample is built with CONFIG_BOOTLOADER_MCUBOOT=y, the sample will
mark the image in slot 1 as pending after an image download.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>