usb/class/usb_dfu: use flash_map data instead of macro labels
usb/class/usb_dfu: use flash_map data instead of macro labels

Now flash_map is proper place to obtain all information about
flash-areas boundaries.

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

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