Support the Tilera platform

This patch provides build and test script updates for the Tilera
platform.  The cmake scripts autodetect if you are configuring on a
system with a Tilera MDE installed, so you can use the normal `cmake`
command in a build directory with standard cmake options.  Note that
we turn off building the documentation on Tilera, even if Sphinx is
installed.

The major modification to our standard workflow is testing: replace
`make test` with `./tile-test`.  This assumes there is a working
connection to a Tilera device.

Installation requires sudo access, i.e., `sudo make install`. If you
intend to use these files regularly, then you should use an installation
prefix that includes `$TILERA_ROOT/tile`.  One good example is
`$TILERA_ROOT/tile/usr/local`, but you can use any directory you want
that is supported by the various `tile-monitor` upload and mount
options.