Replace rpi-image-gen with ClockworkPi-pi-gen for uConsole image builds

[Copilot]

Migrates from Raspberry Pi Foundation's rpi-image-gen (YAML config-based) to ClockworkPi's pi-gen fork (stage-based), which includes uConsole-specific optimizations.

Changes

Submodule

• Replaced rpi-image-gen submodule with pi-gen from https://github.com/ak-rex/ClockworkPi-pi-gen

Build system

• scripts/generate_rpi_image.sh: Rewritten to use pi-gen's stage-based approach
  • Generates dynamic custom stages for kernel installation and uConsole hardware config
  • Supports both prebuilt and custom-built kernels
  • Handles desktop environment selection (GNOME, KDE, MATE, Xfce, etc.)

CI/CD

• .github/workflows/build-and-release.yml: Updated Debian builds (bookworm/trixie) to use pi-gen
  • Creates custom stages: stage2/06-uconsole-kernel, stage2/07-uconsole-config, stage2/08-desktop-environment
  • Ubuntu (jammy) continues manual customization (pi-gen doesn't support Ubuntu)

Documentation

• Updated README.md and IMPLEMENTATION.md with pi-gen stage-based architecture
• Added scripts/pi_gen/README.md with integration details
• Removed obsolete scripts/rpi_image_gen/README.md

Architecture shift

Before: YAML configs → minimal image → manual chroot customization

After: Stage-based builds → complete image from bootstrap

pi-gen provides better Debian ecosystem integration and includes ClockworkPi hardware optimizations out of the box.

Original prompt

Replace raspi-image-gen module and procedures with the proper ones from this other repository:

https://github.com/ak-rex/ClockworkPi-pi-gen

READ THE README: https://github.com/ak-rex/ClockworkPi-pi-gen/blob/main/README.md

Do these steps instead of the ones of raspi-image-gen: replace them for this new.

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