doc: x86: Modify x86 TOC include

Modify toctree glob to exclude common directory which keeps rst
helpers. This allows to include several boards documentation from the
common board directory.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>

boards/x86/index.rst

@@ -7,4 +7,4 @@ x86 Boards
    :maxdepth: 1
    :glob:
 
-   **/index
+   [!common]*/**/*