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/boot/cmdline.txt

@@ -1 +0,0 @@
-usb-storage.quirks=152d:0562:u,14cd:121c:u,0781:cfcb:u console=serial0,115200 console=tty1 root=PARTUUID=cb15ae4d-02 rootfstype=ext4 fsck.repair=yes rootwait cgroup_enable=cpuset cgroup_memory=1 cgroup_enable=memory boot=startos

build/image-recipe/raspberrypi/squashfs/boot/config.sh

@@ -27,20 +27,18 @@ disable_overscan=1
 # (e.g. for USB device mode) or if USB support is not required.
 otg_mode=1
 
-[all]
-
 [pi4]
 # Run as fast as firmware / board allows
 arm_boost=1
-kernel=vmlinuz-${KERNEL_VERSION}-rpi-v8
-initramfs initrd.img-${KERNEL_VERSION}-rpi-v8 followkernel
-
-[pi5]
-kernel=vmlinuz-${KERNEL_VERSION}-rpi-2712
-initramfs initrd.img-${KERNEL_VERSION}-rpi-2712 followkernel
 
 [all]
 gpu_mem=16
 dtoverlay=pwm-2chan,disable-bt
 
-EOF
+# Enable UART for U-Boot and serial console
+enable_uart=1
+
+# Load U-Boot as the bootloader (GRUB is chainloaded from U-Boot)
+kernel=u-boot.bin
+
+EOF

build/image-recipe/raspberrypi/squashfs/boot/config.txt

@@ -84,4 +84,8 @@ arm_boost=1
 gpu_mem=16
 dtoverlay=pwm-2chan,disable-bt
 
-auto_initramfs=1
+# Enable UART for U-Boot and serial console
+enable_uart=1
+
+# Load U-Boot as the bootloader (GRUB is chainloaded from U-Boot)
+kernel=u-boot.bin