doom-wad: split out and fix installation of Doom shareware wad file

This patch splits the installation of the shareware WAD file into a separate
package. The wad file will also be installed into /usr/share/games/doom
directory, since the client will search this directory when starting.

The Makefile rule has also been altered to use tools defined by Buildroot to
extract and install the wad file. The package is also multiple project safe
now.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>