Generates a per-build Buildroot defconfig that layers nbpr-specific settings on top of the active Nerves system's defconfig.

Layered on top in order:

1. The system's defconfig verbatim (e.g. nerves_system_rpi4/nerves_defconfig).
2. BR2_PER_PACKAGE_DIRECTORIES=y so per-package builds don't contend.
3. BR2_PACKAGE_BUSYBOX_SHOW_OTHERS=y so packages overlapping busybox applets (e.g. kmod's tools) aren't dropped as unmet dependencies.
4. BR2_PRIMARY_SITE pointing at Buildroot's source archive — see below.
5. One line per kconfig symbol gating a nested package — see below.
6. BR2_PACKAGE_<UPPER_BR_NAME>=y to enable the target package.
7. One line per resolved build_opt whose schema declared a :br_flag extension, formatted as <br_flag>=<value> with BR-style boolean, string, and integer encoding.

The result is a defconfig file ready to be loaded with make defconfig — but typically we'd write it to O=<dir>/.config directly and follow with make olddefconfig to resolve any dependencies.

Nested packages and their gating symbols

Most Buildroot packages live at package/<name>/, and BR2_PACKAGE_<NAME>=y is all it takes to select one. Some live one level deeper, at package/<parent>/<name>/, and their Config.in is sourced from the parent's inside an if block — fftw-single under fftw, every xlib_* and xdriver_* under x11r7, the qt5*/qt6* and gst1-* families.

For those, BR2_PACKAGE_<NAME>=y on its own is silently discarded by make olddefconfig: the symbol's dependencies aren't met, so kconfig drops it, the package never builds, and the failure only surfaces later as a missing per-package output directory.

The gating symbols can't be derived from the parent directory's name — package/x11r7/ is gated by BR2_PACKAGE_XORG7, not BR2_PACKAGE_X11R7, and package/opengl/ sources its children with no gate at all. So they're read from the tree: find the line in the parent's Config.in that sources this package's Config.in, and take the if conditions enclosing it, outermost first. Nesting is real — the modular Xorg drivers sit inside if BR2_PACKAGE_XORG7 and if BR2_PACKAGE_XSERVER_XORG_SERVER_MODULAR.

Only bare symbol conditions are emitted. A compound condition (Buildroot has one, an || over two freescale-imx platform choices) is a choice between alternatives that nbpr has no basis to make, so it's left alone.

Top-level packages are deliberately excluded from this lookup even though package/Config.in has if blocks of its own: they already build, and re-deriving their gates would change what every existing package's build sees for no benefit.

Why the primary site is set

BR2_BACKUP_SITE defaults to Buildroot's own source archive, which mirrors every tarball mainline Buildroot references. Nerves systems override it to https://dl.nerves-project.org, which only carries what a Nerves system itself builds — reasonable for them, useless here, because nbpr's whole purpose is building packages Nerves systems don't include.

Left alone, an nbpr build's only real source is the package's own upstream site, and some of those are unreliable: gpsd's is a Savannah mirror over plain HTTP, which times out often enough from CI runners to fail roughly half a nine-target matrix, with dl.nerves-project.org then returning 403.

Setting BR2_PRIMARY_SITE to the archive fixes that without touching the system's backup choice. Buildroot tries the primary site first and falls back to the package's own site, so downloads get faster and more reliable at the same time, and upstreams see less traffic — which is what Buildroot recommends the option for.