build-px4-linux

# Build PX4 on Linux ## Inputs - A make target, for example `px4_fmu-v6x_default` or `px4_sitl`. Ask for it if missing; validate it against `boards/`. Build the current checkout as-is, worktree or not. Do not create a worktree or switch branches to build. ## Prepare 1. `arm-none-eabi-gcc --version` must succeed for NuttX targets. If it does not, the host needs `Tools/setup/ubuntu.sh` (Ubuntu) — tell the user rather than installing it, or build in the container: `docker run --rm -w "$PWD" --user "$(id -u):$(id -g)" -v "$PWD:$PWD" px4io/px4-dev:v1.17.0 make <target>`. `Tools/docker_run.sh` passes `-it` and fails without a TTY. 2. The build initializes submodules itself; do not overwrite locally modified ones. ## Build ```bash set -o pipefail make <target> 2>&1 | tail -n 40 ``` Report failures with the output; never treat an existing artifact as evidence of success. After changing a defconfig or Kconfig option, or switching toolchains, `rm -rf build/<target>` first: a stale build dir recompiles nothing and links mixed objects. Do not run a NuttX build concurrently with `make tests` or SITL in the same tree. ## Report The artifact is `build/<target>/<target>.px4` (NuttX) or `build/px4_sitl_default/bin/px4` (SITL). Report the ref, commit, dirty state, target, and artifact path. Building is not flashing; never upload without a separate request.