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. BR2_PACKAGE_<UPPER_BR_NAME>=y to enable the target package.
6. 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.

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.