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.

ports/doom/build.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+if [ -z $BANAN_ARCH ]; then
+	echo  "You must set the BANAN_ARCH environment variable" >&2
+	exit 1
+fi
+
+if [ -z $BANAN_SYSROOT ]; then
+	echo  "You must set the BANAN_ARCH environment variable" >&2
+	exit 1
+fi
+
+CURRENT_DIR=$(dirname $(realpath $0))
+pushd $CURRENT_DIR >/dev/null
+
+if [ ! -d "doomgeneric" ]; then
+	git clone https://github.com/ozkl/doomgeneric.git
+	cd doomgeneric
+	git checkout 613f870b6fa83ede448a247de5a2571092fa729d
+	for patch in ../patches/*; do
+		git am "$patch"
+	done
+	cd ..
+
+	grep -qxF doom ../installed || echo doom >> ../installed
+fi
+
+make --directory doomgeneric/doomgeneric --file Makefile.banan_os
+cp "doomgeneric/doomgeneric/build-${BANAN_ARCH}/doomgeneric" "${BANAN_SYSROOT}/bin/doom"
+
+popd >/dev/null