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/raspberrypi/squashfs/etc/default/grub.d/raspberrypi.cfg

@@ -0,0 +1,4 @@
+# Raspberry Pi-specific GRUB overrides
+# Overrides GRUB_CMDLINE_LINUX from /etc/default/grub with Pi-specific
+# console devices and hardware quirks.
+GRUB_CMDLINE_LINUX="boot=startos console=serial0,115200 console=tty1 usb-storage.quirks=152d:0562:u,14cd:121c:u,0781:cfcb:u cgroup_enable=cpuset cgroup_memory=1 cgroup_enable=memory"

build/image-recipe/raspberrypi/squashfs/etc/startos/config.yaml

@@ -1,6 +1,3 @@
-os-partitions:
-  boot: /dev/mmcblk0p1
-  root: /dev/mmcblk0p2
 ethernet-interface: end0
 wifi-interface: wlan0
 disable-encryption: true