nk3: Add more nrf52 bootloader functionality

[robin-nitrokey]

To replace nrfutil, we should add more NRF52 bootloader functionality to nitrokey-sdk-py and/or pynitrokey.

• [ ] create and sign firmware image

[daringer]

from the nk3-fw repo:

• https://github.com/Nitrokey/nitrokey-3-firmware/blob/main/utils/nrf-bootloader/generate_key.sh

  nrfutil keys generate ${privkey}
  nrfutil keys display --key pk --format code ${privkey} --out_file ${pubkey}
  nrfutil keys display --key pk --format pem ${privkey} --out_file ${pubkey_pem}

• https://github.com/Nitrokey/nitrokey-3-firmware/blob/main/utils/nrf-bootloader/upload.sh

  nrfutil dfu usb-serial -pkg $UPDATE_FILENAME -p $SERIAL_PORT

• https://github.com/Nitrokey/nitrokey-3-firmware/blob/main/utils/nrf-bootloader/sign.sh

  nrfutil pkg generate --hw-version $HW_VERSION --application-version $APP_VERSION --application $APP_HEX_FILE --sd-req $SD_VERSION --key-file $KEY_PATH --app-boot-validation VALIDATE_ECDSA_P256_SHA256 $OUTPUT_FILE

• https://github.com/Nitrokey/nitrokey-3-firmware/blob/main/utils/nrf-bootloader/sign-bootloader.sh

  nrfutil pkg generate --hw-version $HW_VERSION --bootloader-version $BOOTLOADER_VERSION --bootloader $BOOTLOADER_HEX_FILE --sd-req $SD_VERSION --key-file $KEY_PATH  $OUTPUT_FILE