doc: board: Add Google Chromebook Link board doc

This extracts Google Chromebook Link board specific information from
README.x86, converts plain text documentation to reST format and
adds it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index 99b5d2f..59f745d 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -6,4 +6,5 @@
 .. toctree::
    :maxdepth: 2
 
+   google/index
    intel/index