documentation: Update details about external toolchains

    <ul>

      <li>Select the <code>External binary toolchain</code> toolchain

      type</li>

      <li>Select the appropriate <code>External toolchain C

      library</code></li>

      <li>Select the appropriate values for <code>Enable large

      file</code>, <code>Enable IPv6</code>, <code>Enable

      RPC</code>, <code>Enable toolchain

      locale/i18n</code>, <code>Enable WCHAR</code>, <code>Enable

      program invocation</code>, <code>Build/install c++ compiler and

      libstdc++</code>, according to the configuration of your

      external toolchain. Buildroot will check those values at the

      beginning of the compilation process and will tell you if you

      used incorrect values.</li>

      <li>Adjust the <code>External toolchain path</code>

      appropriately. It should be set to a path where a bin/ directory

      contains your cross-compiling tools</li>

      correspond to your cross-compiling tools</li>

    </ul>

    <p>If you are using an external toolchain based on <i>uClibc</i>, the

    <code>Core C library from the external toolchain</code> and

    <code>Libraries to copy from the external toolchain</code> options

    should already have correct values. However, if your external

    toolchain is based on <i>glibc</i>, you'll have to change these values

    according to your cross-compiling toolchain.</p>

    <p>To generate external toolchains, we recommend using

    <a href="http://ymorin.is-a-geek.org/dokuwiki/projects/crosstool">Crosstool-NG</a>.

    It allows generating toolchains based on <i>uClibc</i>, <i>glibc</i>