Update documentation about external toolchain

    </ul>

    <p>Buildroot supports using existing toolchains through a

    mechanism called <i>external toolchain</i>.</p>

    mechanism called <i>external toolchain</i>. The external toolchain

    mechanism is enabled in the <code>Toolchain</code> menu, by

    selecting <code>External toolchain</code> in <code>Toolchain

    type</code>.</p>

    <p>To enable the use of an external toolchain, go to the

    <code>Toolchain</code> menu, and :</p>

    <p>Then, you have three solutions to use an external

    toolchain:</p>

    <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>

      <li>Adjust the <code>External toolchain prefix</code> so that the

      prefix, suffixed with <code>-gcc</code> or <code>-ld</code> will

      correspond to your cross-compiling tools</li>

      <li>Use a predefined external toolchain profile, and let

      Buildroot download, extract and install the toolchain. Buildroot

      already knows about a few CodeSourcery toolchains for ARM,

      PowerPC, MIPS and SuperH. Just select the toolchain profile

      in <code>Toolchain</code> through the available ones. This is

      definitely the easiest solution.</li>

      <li>Use a predefined external toolchain profile, but instead of

      having Buildroot download and extract the toolchain, you can

      tell Buildroot where your toolchain is already installed on your

      system. Just select the toolchain profile

      in <code>Toolchain</code> through the available ones,

      unselect <code>Download toolchain automatically</code>, and fill

      the <code>Toolchain path</code> text entry with the path to your

      cross-compiling toolchain.</li>

      <li>Use a completely custom external toolchain. This is

      particularly useful for toolchains generated using

      Crosstool-NG. To do this, select the <code>Custom

      toolchain</code> solution in the <code>Toolchain</code>

      list. You need to fill the <code>Toolchain

      path</code>, <code>Toolchain prefix</code> and <code>External

      toolchain C library</code> options. Then, you have to tell

      Buildroot what your external toolchain supports. If your

      external toolchain uses the <i>glibc</i> library, you only have

      to tell whether your toolchain supports C++ or not. If your

      external toolchain uses the <i>uclibc</i> library, then you have

      to tell Buildroot if it supports largefile, IPv6, RPC,

      wide-char, locale, program invocation, threads and C++. At the

      beginning of the execution, Buildroot will tell you if the

      selected options do not match the toolchain configuration.</li>

    </ul>

    <p>Our external toolchain support has been tested with toolchains