usb/class/usb_dfu: use predefined string for image descriptors
usb/class/usb_dfu: use predefined string for image descriptors

Use predefined string for images string descriptors instead
of from DTS build-time generated labels.
This labels will be deprecated.

String content are kept the same as before.

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

galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,ldts/zephyr,ldts/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,explora26/zephyr