feat: Add U-Boot support and related commands to Axvisor

- Introduced new command variants for U-Boot and defconfig in the Axvisor CLI.
- Implemented `ArgsUboot` and `ArgsDefconfig` structs for parsing U-Boot and defconfig commands.
- Enhanced `Axvisor` struct to handle U-Boot commands and configurations.
- Created `AxvisorUbootSnapshot` to manage U-Boot specific configurations.
- Updated `AppContext` to include `axvisor_dir` for better path management.
- Added tests for U-Boot command parsing and functionality.
- Refactored existing code to accommodate new U-Boot features and ensure backward compatibility.
- Updated repository references in `repos.csv` for new dependencies.

Author: ZCShou