topology: Add doc for including other files in the text conf file

topology: Add doc for including other files in the text conf file

This patch adds document about how to include other files in the text
configuration files, by alsaconf syntax <xxx> and <searchdir:xxx>.

Users may define common info in separate files (e.g. vendor tokens,
tuples) and share them for different platforms, by including them via
syntax <path/to/configuration-file>. This can save the total size of
files. Users can also specifiy additional configuraiton directories
relative to "/usr/share/alsa/" to search the included files, via syntax
<searchdir:relative-path/to/usr/share/alsa>.

The alsaconf will search and open an included file in the following order
of priority:
1. directly open the file by its name;
2. search for the file name in "/usr/share/alsa";
3. search for the file name in user specified subdirectories under
   "/usr/share/alsa".

The order of the included files need not to be same as their dependencies,
because the toplogy library will load all of them before parsing their
dependencies.

Signed-off-by: Mengdong Lin <mengdong.lin@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>