ON RASPBERRY
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sudo apt-get install libQt4-dev libusb-1.0-0-dev python-dev rsync

ON HOST
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export RASPI=192.168.1.17
export RASCROSS_DIR="$HOME/raspberrypi"
export HYPERION_DIR="$HOME/hyperion"

# install required packages
sudo apt-get install git rsync cmake build-essential libQt4-dev libusb-1.0-0-dev python-dev
sudo apt-get install git rsync cmake ia32-libs protobuf-compiler

# On newer version of Ubuntu (and maybe other distros) the ia32-libs is not available, the following
# install solved this for me (TODO: verify what is really required)
sudo apt-get install libc6:i386 libgcc1:i386 gcc-4.6-base:i386 libstdc++5:i386 libstdc++6:i386

# create the rootfs by copying it from an rpi running for example raspbmc
mkdir -p "$RASCROSS_DIR/rootfs"
rsync -rl --delete-after --safe-links pi@$RASPI:/{lib,usr} "$RASCROSS_DIR/rootfs"

# get the raspberry pi firmware and add it to the rootfs
git clone https://github.com/raspberrypi/firmware.git "$RASCROSS_DIR/firmware"
ln -s "$RASCROSS_DIR/firmware/hardfp/opt" "$RASCROSS_DIR/rootfs/opt"

# get the compile tools
git clone git://github.com/raspberrypi/tools.git "$RASCROSS_DIR/tools"

# get the Hyperion sources
git clone --recursive https://github.com/tvdzwan/hyperion.git "$HYPERION_DIR"

# if you forget the --resursive in above statement or you are updating an existing clone you need
# to clone the protobuf submodule by runnning the follwing two statements:
git submodule init
git submodule update

# do a native build (to build the protobuf compiler for the native platform)
mkdir "$HYPERION_DIR/build"
cmake -DENABLE_DISPMANX=OFF --build "$HYPERION_DIR/build" "$HYPERION_DIR"

# do the rpi build 
# specify the protoc export file to import the protobuf compiler from the native build
mkdir "$HYPERION_DIR/build-rpi"
cmake -DCMAKE_TOOLCHAIN_FILE="$HYPERION_DIR/Toolchain-RaspberryPi.cmake" -DIMPORT_PROTOC=$HYPERION_DIR/build/protoc_export.cmake --build "$HYPERION_DIR/build-rpi" "$HYPERION_DIR"

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These instructions are based on the guide given by:
http://airwiki.ws.dei.polimi.it/index.php/Cross-compiling_for_the_RaspberryPi