Now ports define some environment variables, and call a unified bash
script that handles configuration, recompilation and installation.
If a port uses non-configure buildsystem, port script can also specify
custom build and install step.
This patch contains simple infrastructure for porting software for
banan-os. I added a doom as the first port. Doom needs a wad file
that you have to aquire yourself. I am not sure if I am allowed to
redistribute doom1.wad (shareware) version so I decided not to.