Migrate to mbt build system

[mgrossmann]

Replace legacy config.mk/rules.mk/scripts build system with mbt.

• Add mbt as git submodule
• Create project.toml (library, autocall=false, crent370 dependency)
• New 2-line Makefile
• New .env.example with MBT_MVS_* variables
• Add CI workflows (build.yml, release.yml)
• Update .gitignore for mbt artifacts
• Remove old config.mk, rules.mk, scripts/, contrib/crent370_sdk
• Clean up .s and .o build artifacts from src/