buildroot/docs/manual/customize-configuration.txt

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[[customize-store-basics]]
=== Basics for storing the configuration

When you have a buildroot configuration that you are satisfied with and
you want to share it with others, put it under revision control or move
on to a different buildroot project, you need to store the configuration
so it can be rebuilt later. The configuration that needs to be stored
consists of the buildroot configuration, the configuration files for
packages that you use (kernel, busybox, uClibc, ...), and your rootfs
modifications.

[[customize-store-buildroot-config]]
==== Buildroot configuration

For storing the buildroot configuration itself, buildroot offers the
following command: +make savedefconfig+.

This strips the buildroot configuration down by removing configuration
options that are at their default value. The result is stored in a file
called +defconfig+.  If you want to save it in another place, change
the +BR2_DEFCONFIG+ option, or call make with +make savedefconfig
BR2_DEFCONFIG=<path-to-defconfig>+.  The usual place is
+configs/<boardname>_defconfig+. The configuration can then be rebuilt by
running +make <boardname>_defconfig+.

Alternatively, you can copy the file to any other place and rebuild with
+make defconfig BR2_DEFCONFIG=<path-to-defconfig-file>+.


[[customize-store-package-config]]
==== Other package configuration

The configuration files for busybox, the linux kernel, barebox and
uClibc should be stored as well if changed. For each of these, a
buildroot configuration option exists to point to an input
configuration file, e.g. +BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE+.  To
save their configuration, set those configuration options to a path
outside your output directory,
e.g. +board/<manufacturer>/<boardname>/linux.config+.  Then, copy the
configuration files to that path.

Make sure that you create a configuration file 'before' changing
the +BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE+ etc. options.  Otherwise,
buildroot will try to access this config file, which doesn't exist
yet, and will fail. You can create the configuration file by running
+make linux-menuconfig+ etc.

Buildroot provides a few helper targets to make the saving of
configuration files easier.

* +make linux-update-defconfig+ saves the linux configuration to the
  path specified by +BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE+.  It
  simplifies the config file by removing default values.  However,
  this only works with kernels starting from 2.6.33.  For earlier
  kernels, use +make linux-update-config+.
* +make busybox-update-config+ saves the busybox configuration to the
  path specified by +BR2_PACKAGE_BUSYBOX_CONFIG+.
* +make uclibc-update-config+ saves the uClibc configuration to the
  path specified by +BR2_UCLIBC_CONFIG+.
* +make barebox-update-defconfig+ saves the barebox configuration to the
  path specified by +BR2_TARGET_BAREBOX_CUSTOM_CONFIG_FILE+.
* For at91bootstrap3, no helper exists so you have to copy the config
  file manually to +BR2_TARGET_AT91BOOTSTRAP3_CUSTOM_CONFIG_FILE+.