title: Buildroot - extract a custom board/buildroot config/kernel config out of tree
body: <p>I have customized buildroot with the new board ( derived from raspberry pi zero ). So my changes are (in-tree):</p>
  
  <pre><code>.config
  board/passkeeper/genimage-passkeeper.cfg
  board/passkeeper/post-build.sh
  board/passkeeper/post-image.sh
  board/passkeeper/rootfs_overlay/etc/init.d/S41passkeeper
  board/passkeeper/rootfs_overlay/etc/mdev.conf
  board/passkeeper/rootfs_overlay/etc/udhcpd.conf
  configs/passkeeper_defconfig
  output/build/linux-custom/.config
  </code></pre>
  
  <p>Now, reading the documentation - I am a bit confused on how to put all these things into the separate folder via <code>BR2_EXTERNAL</code>. Also I'm not sure how do I move the linux configuration from <code>output/build/linux-custom/.config</code></p>
  
  <pre><code>make linux-update-defconfig BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE=/tmp/passkeeper/linux/linux-config
  </code></pre>
  
  <p>results in </p>
  
  <pre><code>Unable to perform linux-update-defconfig when using a defconfig rule
  </code></pre>
  
  <p>Can somebody please provide step-by-step guide on that?</p>