feat: raspberry pi U-Boot + GPT + btrfs boot chain

Switch Raspberry Pi builds from proprietary firmware direct-boot to a
firmware → U-Boot → GRUB → kernel chain using GPT partitioning:

- GPT partition layout with fixed UUIDs matching os_install: firmware
  (128MB), ESP (100MB), boot (2GB FAT32), root (btrfs)
- U-Boot as the kernel in config.txt, chainloading GRUB EFI
- Pi-specific GRUB config overrides (console, USB quirks, cgroup)
- Btrfs root with shrink-to-minimum for image compression
- init_resize.sh updated for GPT (sgdisk -e) and btrfs resize
- Removed os-partitions from config.yaml (now derived from fstab)

build/image-recipe/Dockerfile

@@ -23,6 +23,8 @@ RUN apt-get update && \
     squashfs-tools \
     rsync \
     b3sum \
+    btrfs-progs \
+    gdisk \
     dpkg-dev