Ports: Add doom port

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.

script/build.sh

@@ -49,6 +49,8 @@ build_toolchain () {
 create_image () {
 	build_target bootloader
 	build_target install-sysroot
+	$BANAN_ROOT_DIR/ports/build.sh
+	build_target package-sysroot
 	$BANAN_SCRIPT_DIR/image.sh "$1"
 }