fix: RPi image uses MBR and firmware on boot partition

- Switch from GPT to MBR (dos) partition table — GPT + autoboot.txt
  fails on many Pi 4 EEPROM versions
- Copy firmware blobs (start*.elf, fixup*.dat) to partition 1 (KSOLOCTL)
  so the EEPROM can find and load them
- Increase boot control partition from 16 MB to 32 MB to fit firmware
- Mark partition 1 as bootable

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build/scripts/create-rpi-image.sh

@@ -1,15 +1,17 @@
 #!/bin/bash
 # create-rpi-image.sh — Create a raw disk image for Raspberry Pi SD card
 #
-# Partition layout (GPT):
-#   Part 1: Boot Control  (16 MB, FAT32, label KSOLOCTL) — autoboot.txt only
-#   Part 2: Boot A        (256 MB, FAT32, label KSOLOA)  — firmware + kernel + DTBs + initramfs
+# Partition layout (MBR):
+#   Part 1: Boot Control  (32 MB, FAT32, label KSOLOCTL) — firmware + autoboot.txt
+#   Part 2: Boot A        (256 MB, FAT32, label KSOLOA)  — kernel + DTBs + initramfs
 #   Part 3: Boot B        (256 MB, FAT32, label KSOLOB)  — same as Boot A (initially identical)
 #   Part 4: Data          (remaining of 2GB, ext4, label KSOLODATA)
 #
-# The RPi uses autoboot.txt in the control partition to implement A/B boot
-# via the tryboot mechanism (tryboot_a_b=1). Normal boot → partition 2 (Boot A),
-# tryboot → partition 3 (Boot B).
+# The RPi EEPROM loads start4.elf from partition 1 (KSOLOCTL).
+# autoboot.txt on partition 1 redirects boot_partition to 2 (Boot A) or 3 (Boot B).
+# The firmware then loads config.txt, kernel, and initramfs from the selected partition.
+#
+# MBR is required — GPT + autoboot.txt is not reliably supported on Pi 4.
 #
 # Usage: build/scripts/create-rpi-image.sh
 set -euo pipefail
@@ -64,22 +66,24 @@ mkdir -p "$OUTPUT_DIR"
 # --- Create sparse image ---
 dd if=/dev/zero of="$IMG_OUTPUT" bs=1M count=0 seek="$IMG_SIZE_MB" 2>/dev/null
 
-# --- Partition table (GPT) ---
-# Part 1: Boot Control  16 MB FAT32
-# Part 2: Boot A       256 MB FAT32
-# Part 3: Boot B       256 MB FAT32
+# --- Partition table (MBR) ---
+# MBR is required for reliable RPi boot with autoboot.txt.
+# GPT + autoboot.txt fails on many Pi 4 EEPROM versions.
+# Part 1: Boot Control  32 MB FAT32 (firmware + autoboot.txt)
+# Part 2: Boot A       256 MB FAT32 (kernel + initramfs + DTBs)
+# Part 3: Boot B       256 MB FAT32 (kernel + initramfs + DTBs)
 # Part 4: Data         remaining ext4
 sfdisk "$IMG_OUTPUT" << EOF
-label: gpt
+label: dos
 
-# Boot Control partition: 16 MB
-start=2048, size=32768, type=EBD0A0A2-B9E5-4433-87C0-68B6B72699C7, name="BootCtl"
-# Boot A partition: 256 MB
-size=524288, type=EBD0A0A2-B9E5-4433-87C0-68B6B72699C7, name="BootA"
-# Boot B partition: 256 MB
-size=524288, type=EBD0A0A2-B9E5-4433-87C0-68B6B72699C7, name="BootB"
-# Data partition: remaining
-type=0FC63DAF-8483-4772-8E79-3D69D8477DE4, name="Data"
+# Boot Control partition: 32 MB, FAT32 (type 0c = W95 FAT32 LBA)
+start=2048, size=65536, type=c, bootable
+# Boot A partition: 256 MB, FAT32
+size=524288, type=c
+# Boot B partition: 256 MB, FAT32
+size=524288, type=c
+# Data partition: remaining, Linux
+type=83
 EOF
 
 # --- Set up loop device ---
@@ -146,7 +150,9 @@ mount "$P3" "$MNT_BOOTB"
 mount "$P4" "$MNT_DATA"
 
 # --- Boot Control Partition (KSOLOCTL) ---
-echo "    Writing autoboot.txt..."
+# The RPi EEPROM loads start4.elf from partition 1.
+# autoboot.txt tells the firmware which partition has config.txt + kernel.
+echo "    Writing autoboot.txt + firmware to boot control partition..."
 cat > "$MNT_CTL/autoboot.txt" << 'AUTOBOOT'
 [all]
 tryboot_a_b=1
@@ -155,6 +161,17 @@ boot_partition=2
 boot_partition=3
 AUTOBOOT
 
+# Copy firmware blobs — REQUIRED on partition 1 for EEPROM to boot
+if ls "$RPI_FIRMWARE_DIR"/start*.elf 1>/dev/null 2>&1; then
+    cp "$RPI_FIRMWARE_DIR"/start*.elf "$MNT_CTL/"
+fi
+if ls "$RPI_FIRMWARE_DIR"/fixup*.dat 1>/dev/null 2>&1; then
+    cp "$RPI_FIRMWARE_DIR"/fixup*.dat "$MNT_CTL/"
+fi
+if [ -f "$RPI_FIRMWARE_DIR/bootcode.bin" ]; then
+    cp "$RPI_FIRMWARE_DIR/bootcode.bin" "$MNT_CTL/"
+fi
+
 # --- Helper: populate a boot partition ---
 populate_boot_partition() {
     local MNT="$1"
@@ -183,17 +200,6 @@ CFGTXT
     # Copy initramfs
     cp "$INITRAMFS" "$MNT/kubesolo-os.gz"
 
-    # Copy firmware blobs (start*.elf, fixup*.dat)
-    if ls "$RPI_FIRMWARE_DIR"/start*.elf 1>/dev/null 2>&1; then
-        cp "$RPI_FIRMWARE_DIR"/start*.elf "$MNT/"
-    fi
-    if ls "$RPI_FIRMWARE_DIR"/fixup*.dat 1>/dev/null 2>&1; then
-        cp "$RPI_FIRMWARE_DIR"/fixup*.dat "$MNT/"
-    fi
-    if [ -f "$RPI_FIRMWARE_DIR/bootcode.bin" ]; then
-        cp "$RPI_FIRMWARE_DIR/bootcode.bin" "$MNT/"
-    fi
-
     # Copy DTB overlays
     if [ -d "$RPI_FIRMWARE_DIR/overlays" ]; then
         cp -r "$RPI_FIRMWARE_DIR/overlays" "$MNT/"
@@ -225,9 +231,9 @@ sync
 echo ""
 echo "==> Raspberry Pi disk image created: $IMG_OUTPUT"
 echo "    Size: $(du -h "$IMG_OUTPUT" | cut -f1)"
-echo "    Part 1 (KSOLOCTL):  Boot control (autoboot.txt)"
-echo "    Part 2 (KSOLOA):    Boot A — firmware + kernel + initramfs"
-echo "    Part 3 (KSOLOB):    Boot B — firmware + kernel + initramfs"
+echo "    Part 1 (KSOLOCTL):  Firmware + autoboot.txt (boot control)"
+echo "    Part 2 (KSOLOA):    Boot A — kernel + initramfs + DTBs"
+echo "    Part 3 (KSOLOB):    Boot B — kernel + initramfs + DTBs"
 echo "    Part 4 (KSOLODATA): Persistent K8s state"
 echo ""
 echo "Write to SD card with:"