Customizing Buildroot Root Filesystems with Overlays and Post-Build Scripts

Hello again. In the previous lesson, you traced Buildroot artifacts from source cache through output/build, host, staging, target, and finally images. That map now gives us the right insertion point for filesystem customization: an overlay and post-build script act on the target filesystem tree before Buildroot turns it into a deployable image.

In this lesson, you will add product-owned configuration files with a skeleton overlay, then use a post-build script to generate one controlled file and normalize the timestamps of the files it owns. The goal is not merely to make a QEMU image boot with a custom banner; it is to establish a maintainable pattern for the automotive/industrial gateway prototype: static configuration is explicit, generated configuration has declared inputs, and no build host details, secrets, or wall-clock time leak into the target filesystem.

Two complementary mechanisms, with different responsibilities

A Buildroot configuration selects packages and system-wide settings, but a deployable product also needs files that do not naturally belong to any package: product defaults, site-wide configuration, legal notices, small system policies, and board-level settings.

Buildroot recommends two mechanisms that can coexist:

1. A root filesystem overlay is a directory tree copied onto the completed target filesystem tree.
2. A post-build script runs after selected software has been installed into that tree and before filesystem images are assembled.