Build: Add USB boot + NVMe HAT support

- --target flag in build.sh (sd|usb|nvme)
- NVMe kernel driver enabled for --target nvme
- EEPROM boot-order config in first-boot wizard
- USB/NVMe flashing docs in build/README.md
- Hardware compatibility + user manual docs from P7

build/build.sh

@@ -1,16 +1,19 @@
 #!/bin/bash
-# build.sh — Build RPi Audio Mixer SD card image from scratch
+# build.sh — Build RPi Audio Mixer SD card / USB / NVMe image from scratch
 #
-# Produces a ready-to-flash Raspberry Pi 4B SD card image with:
+# Produces a ready-to-flash Raspberry Pi 4B disk image with:
 #   - RPiOS Lite (64-bit, Bookworm) base
 #   - Optional PREEMPT_RT kernel (cross-compiled on x86)
 #   - JACK2, Carla, ALSA tools, Python deps
 #   - Full mixer engine + web UI + Kivy touch UI
 #   - First-boot setup wizard
 #   - systemd services for auto-start
+#   - Target-specific boot config (SD / USB SSD / NVMe HAT)
 #
 # Usage:
-#   ./build.sh                          # Full build (default)
+#   ./build.sh                          # Full build, SD card (default)
+#   ./build.sh --target usb             # Build for USB SSD boot
+#   ./build.sh --target nvme            # Build for NVMe HAT boot
 #   ./build.sh --skip-kernel            # Skip RT kernel build (use stock kernel)
 #   ./build.sh --kernel-only            # Only build RT kernel, skip image
 #   ./build.sh --output ./my-image.img  # Custom output path
@@ -31,7 +34,7 @@
 #   - Cross-compile (optional): gcc-aarch64-linux-gnu, libssl-dev:arm64
 #
 # Output:
-#   build/out/rpi-audio-mixer-<version>.img     SD card image
+#   build/out/rpi-audio-mixer-<version>.img     Disk image (SD/USB/NVMe)
 #   build/out/rpi-audio-mixer-<version>.img.xz  Compressed image
 
 set -euo pipefail
@@ -48,6 +51,9 @@ DOWNLOAD_DIR="$BUILD_DIR/downloads"
 MIXER_VERSION="${MIXER_VERSION:-$(git -C "$PROJECT_ROOT" describe --tags --always 2>/dev/null || echo 'dev')}"
 MIXER_API_KEY="${MIXER_API_KEY:-mixer-$(head -c 12 /dev/urandom | base64 | tr -dc 'a-zA-Z0-9' | head -c 16)}"
 MIXER_HOSTNAME="${MIXER_HOSTNAME:-pi-mixer}"
+
+# Boot target
+TARGET="${TARGET:-sd}"  # sd, usb, or nvme
 MIXER_WIFI_SSID="${MIXER_WIFI_SSID:-}"
 MIXER_WIFI_PASS="${MIXER_WIFI_PASS:-}"
 
@@ -255,6 +261,14 @@ build_rt_kernel() {
         # USB audio (build as module)
         scripts/config -m CONFIG_SND_USB_AUDIO
 
+        # NVMe support (for NVMe HAT boot)
+        if [[ "$TARGET" == "nvme" ]]; then
+            log "Enabling NVMe driver for NVMe HAT target..."
+            scripts/config -m CONFIG_BLK_DEV_NVME
+            scripts/config -e CONFIG_NVME_CORE
+            scripts/config -e CONFIG_NVME_FABRICS
+        fi
+
         make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE" olddefconfig
     else
         log "Kernel .config already has PREEMPT_RT enabled"
@@ -532,9 +546,41 @@ finalize_image() {
         echo "  Kernel:   Stock RPiOS (PREEMPT_RT not included)"
     fi
     echo ""
-    echo -e "${BOLD}To flash to SD card:${NC}"
-    echo "  sudo dd if=$img_file of=/dev/sdX bs=4M status=progress conv=fsync"
-    echo ""
+    echo -e "${BOLD}Boot target:${NC} ${TARGET^^}"
+    if [[ "$TARGET" == "sd" ]]; then
+        echo -e "${BOLD}To flash to SD card:${NC}"
+        echo "  sudo dd if=$img_file of=/dev/mmcblk0 bs=4M status=progress conv=fsync"
+        echo ""
+        echo -e "${BOLD}USB/NVMe boot alternative:${NC}"
+        echo "  Same image works on USB SSD or NVMe — just dd to the right device."
+        echo "  Then configure EEPROM: sudo rpi-eeprom-config --edit"
+        echo "  Set BOOT_ORDER=0xf41 (USB → SD) or 0xf614 (NVMe → USB → SD)"
+        echo "  See docs/build-guide.md §USB Boot for details."
+    elif [[ "$TARGET" == "usb" ]]; then
+        echo -e "${BOLD}To flash to USB SSD:${NC}"
+        echo "  sudo dd if=$img_file of=/dev/sda bs=4M status=progress conv=fsync"
+        echo ""
+        echo -e "${BOLD}After flashing, configure EEPROM on the Pi:${NC}"
+        echo "  sudo rpi-eeprom-config --edit"
+        echo "  Set: BOOT_ORDER=0xf41"
+        echo "  (Boots USB → SD → Restart)"
+        echo ""
+        echo "  Alternatively, the first-boot wizard can do this automatically"
+        echo "  if an SD card with stock RPiOS is used to bootstrap."
+    elif [[ "$TARGET" == "nvme" ]]; then
+        echo -e "${BOLD}To flash to NVMe SSD (via HAT):${NC}"
+        echo "  1. Connect NVMe SSD to HAT"
+        echo "  2. Write image directly:"
+        echo "     sudo dd if=$img_file of=/dev/nvme0n1 bs=4M status=progress conv=fsync"
+        echo ""
+        echo -e "${BOLD}After flashing, configure EEPROM on the Pi:${NC}"
+        echo "  sudo rpi-eeprom-config --edit"
+        echo "  Set: BOOT_ORDER=0xf614"
+        echo "  (Boots NVMe → USB → SD → Restart)"
+        echo ""
+        echo "  Kernel config includes CONFIG_BLK_DEV_NVMe=m"
+        echo ""
+    fi
     echo -e "${BOLD}First boot:${NC}"
     echo "  - The setup wizard will guide you through configuration"
     echo "  - Default hostname: $MIXER_HOSTNAME"