Fix build.sh --target arg parsing, add USB/NVMe boot instructions to docs

build/build.sh

@@ -139,6 +139,7 @@ while [[ $# -gt 0 ]]; do
     case "$1" in
         --skip-kernel)   BUILD_KERNEL=false; shift ;;
         --kernel-only)   KERNEL_ONLY=true; shift ;;
+        --target)        TARGET="$2"; shift 2 ;;
         --output)        OUTPUT_PATH="$2"; shift 2 ;;
         --image-size)    IMAGE_SIZE_MB="$2"; shift 2 ;;
         --version)       MIXER_VERSION="$2"; shift 2 ;;