documentation: clarify Buildroot steps

    <p>This command will generally perform the following steps:</p>

    <ul>

      <li>Download source files (as required)</li>

      <li>Configure cross-compile toolchain</li>

      <li>Build/install cross-compile toolchain</li>

      <li>Configure, build and install the cross-compiling toolchain

      if an internal toolchain is used, or import a toolchain if an

      external toolchain is used</li>

      <li>Build/install selected target packages</li>

      <li>Build a kernel image</li>

      <li>Build a kernel image, if selected</li>

      <li>Build a bootloader image, if selected</li>

      <li>Create a root filesystem in selected formats</li>

    </ul>

    <p>Some of the above steps might not be performed if they are not

    selected in the Buildroot configuration.

   </p>

    <p>Buildroot output is stored in a single directory, <code>output/</code>.

    This directory contains several subdirectories:</p>