Bootstrap script shouldn't do a git pull, should instead warn about existing checkout

[mithro]

Currently the bootstrap script does;

if [ -e HDMI2USB-misoc-firmware ]; then
 cd HDMI2USB-misoc-firmware
 git pull || exit 1
else
 git clone --recurse-submodules $GIT_REPO || exit 1
 cd HDMI2USB-misoc-firmware
 git checkout $GIT_BRANCH || exit 1
fi

It should do;

if [ -e HDMI2USB-misoc-firmware ]; then
  echo "Existing checkout found (see HDMI2USB-misoc-firmware directory), please remove before running."
  exit 1
else
 git clone --recurse-submodules $GIT_REPO || exit 1
 cd HDMI2USB-misoc-firmware
 git checkout $GIT_BRANCH || exit 1
fi