Fix netboot: hardcode config values, simplify boot chain

- Hardcode ROOT_URL and OVERLAYROOT in netboot script
- Remove cmdline parsing that was failing silently
- Simplify boot.ipxe to chain to netboot.ipxe
- Add rebuild-initramfs.sh helper script

Resolves kernel panic caused by cmdline parsing issues.

rebuild-initramfs.sh

@@ -0,0 +1,51 @@
+#!/bin/bash
+# Rebuild initramfs with updated netboot script
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+BUILD_DIR="$SCRIPT_DIR/build"
+ROOTFS="$BUILD_DIR/rootfs"
+HTTP_DIR="$SCRIPT_DIR/http"
+
+if [ ! -d "$ROOTFS" ]; then
+    echo "ERROR: Rootfs not found at $ROOTFS"
+    echo "Run build-image.sh first"
+    exit 1
+fi
+
+echo "=== Copying updated initramfs scripts ==="
+cp "$SCRIPT_DIR/initramfs/scripts/netboot" "$ROOTFS/usr/share/initramfs-tools/scripts/netboot"
+cp "$SCRIPT_DIR/initramfs/hooks/netboot" "$ROOTFS/usr/share/initramfs-tools/hooks/netboot"
+cp "$SCRIPT_DIR/initramfs/initramfs.conf" "$ROOTFS/etc/initramfs-tools/initramfs.conf"
+cp "$SCRIPT_DIR/initramfs/modules" "$ROOTFS/etc/initramfs-tools/modules"
+
+echo "=== Getting kernel version ==="
+KVER=$(ls "$ROOTFS/lib/modules/" | head -1)
+echo "Kernel version: $KVER"
+
+echo "=== Mounting filesystems for chroot ==="
+mount --bind /proc "$ROOTFS/proc"
+mount --bind /sys "$ROOTFS/sys"
+mount --bind /dev "$ROOTFS/dev"
+
+cleanup() {
+    echo "=== Cleaning up mounts ==="
+    umount "$ROOTFS/proc" 2>/dev/null || true
+    umount "$ROOTFS/sys" 2>/dev/null || true
+    umount "$ROOTFS/dev" 2>/dev/null || true
+}
+trap cleanup EXIT
+
+echo "=== Rebuilding initramfs ==="
+chroot "$ROOTFS" mkinitramfs -v -o /boot/initrd-netboot.img "$KVER"
+
+echo "=== Copying to http directory ==="
+cp "$ROOTFS/boot/initrd-netboot.img" "$HTTP_DIR/"
+chmod 644 "$HTTP_DIR/initrd-netboot.img"
+
+echo ""
+echo "=== Done! ==="
+echo "New initramfs: $HTTP_DIR/initrd-netboot.img"
+ls -lh "$HTTP_DIR/initrd-netboot.img"
+echo ""
+echo "Run 'make deploy' to sync to NAS"