Add flatpak bundle build script

* Builds the flatpak manifest (uses a temporary directory for
  intermediate files)
* Bundles the flatpak and writes it to the current directory

packaging/flatpak/build-bundle.sh

@@ -0,0 +1,32 @@
+#!/bin/bash -xe
+SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+
+MANIFEST=com.freerdp.FreeRDP
+
+BUILD_BASE=$(mktemp -d)
+if [ $# -gt 0 ];
+then
+	BUILD_BASE=$1
+fi
+
+echo "Using $BUILD_BASE as temporary build directory"
+REPO=$BUILD_BASE/repo
+BUILD=$BUILD_BASE/build
+STATE=$BUILD_BASE/state
+
+BUILDER=$(which flatpak-builder)
+if [ ! -x "$BUILDER" ];
+then
+	echo "command 'flatpak-builder' could not be found, please install and add to PATH"
+	exit 1
+fi
+
+FLATPAK=$(which flatpak)
+if [ ! -x "$FLATPAK" ];
+then
+	echo "command 'flatpak' could not be found, please install and add to PATH"
+	exit 1
+fi
+
+flatpak-builder -v --repo=$REPO --state-dir=$STATE $BUILD $SCRIPT_DIR/$MANIFEST.json --force-clean
+flatpak build-bundle -v $REPO $MANIFEST.flatpak $MANIFEST --runtime-repo=https://flathub.org/repo/flathub.flatpakrepo