Changes

feat(usdr): add USDR backend support (osmo-trx-usdr)

- Implement USDRDevice backend for Transceiver52M
- Add --with-usdr configure option and build integration
- Add systemd service unit (osmo-trx-usdr.service)
- Add Debian packaging files for osmo-trx-usdr
- Provide example configuration file
- Extend manuals and device documentation with USDR backend section

Change-Id: I5d1d25921514954c4929ae6e7352168b3ceb05df