Simplify dist/iso build flow to bare essentials

[trufae]

Motivation

• Reduce the surface area and maintenance overhead of the ISO builder by removing advanced knobs, platform branching and overengineered plumbing while preserving the basic ISO build path.
• Make the dist/ packaging scripts minimal and easy to reason about so the essential workflow (build radare2 in a container, produce an ISO, run in QEMU) remains available.

Description

• Simplified dist/iso/Makefile to a minimal set of targets (iso, run, clean, help) and a straightforward docker run invocation that mounts the repo and runs the build script.
• Reduced dist/iso/config.mk to only core variables required to build and run the ISO (image, release, arch, output/work paths, git refs, r2pm and QEMU memory).
• Rewrote dist/iso/scripts/build-iso.sh to the essential steps: install minimal deps, configure live-build, add a concise chroot hook that clones and builds radare2, optionally installs r2pm plugins, and copies the produced ISO into output/.
• Shortened dist/iso/README.md to document the simplified usage and output contract for the reduced interface.

Testing

• Ran a shell syntax check on the build script with bash -n dist/iso/scripts/build-iso.sh which completed successfully.
• Verified make help with make -C dist/iso help, which printed the simplified usage successfully.
• Performed a dry-run of the iso target with make -C dist/iso -n iso | head -n 40 to confirm the simplified Docker invocation, which showed the expected command and completed without errors.

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Codex Task