usb/class/usb_dfu: allow to operate only on the primary slot
usb/class/usb_dfu: allow to operate only on the primary slot

It is possible that USB DFU class is used by MCUBoot
itself while it has enabled CONFIG_SINGLE_APPLICATIO_SLOT.
In this mode only application slot is the primary slot.

Adapted DFU class descriptor and services to optionally
serve only for one image.

Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr