usb: dfu: add permanent download and automatic reboot

This commit adds the USB_DFU_PERMANENT_DOWNLOAD and USB_DFU_REBOOT and
symbols.When the permanent download symbol is enabled, slot 1 will be
marked as confirmed. With the reboot symbol enabled, the devices
automatically reboots after the download is completed.

The functionality is split into two symbols to allow the automatic
reboot without confirming the image. This enables image confirmation via
another channel. For example via the shell’s Mcuboot commands.

This functionality allows downloading an image to the device without the
user having to interact with the device. It is useful in cases where
ease of use is more important then safety. For example  when using USB
download for daily development. This is especially applicable for
devices with a closed case.

The changes were tested on an nrf52840dk. The following line can be used
to build the USB DFU example with the symbols enabled.

west build -b nrf52840dk_nrf52840 zephyr/samples/subsys/usb/dfu \
-d build-dfu -- -DCONFIG_BOOTLOADER_MCUBOOT=y \
-DOVERLAY_CONFIG=overlay-reboot-permanent.conf \
-DCONFIG_MCUBOOT_SIGNATURE_KEY_FILE\
=\"bootloader/mcuboot/root-rsa-2048.pem\"

Fixes #41921

Signed-off-by: Martijn Stommels <martijn@martijnpc.nl>