Allow system-files interface for `dfx-mgr` snap


As part of our efforts to enable dynamic FPGA bitstream loading in snap environment, I snapped the dfx-mgr application provided by AMD. dfx-mgr is an application that loads binaries to FPGA and changes device-trees by utilizing configfs. So we need write access to configfs.

By the time of implementation of the snap application, there is no way to use a specific interface designed of this job so we need to use system-files interface with write access to /sys/kernel/config

      - /sys/kernel/config

Since the project is open-source source code can be found here

Looking forward to hear your opinions

Best Regards,