Update SHV menu item to calculate the hash the same way the app does

[amcmanus]

The app now generates a base64 version of the dm-verity hash. Add the xxd tool, along with logic like:

if [[ `which xxd` && `which base64` ]]; then
  base64_hash=$( echo -n ${HASH} \| xxd -r -p \| base64 )
  echo "Base64 Hash for SHV: ${base64_hash}"
  read -p "Press enter once you have recorded the base64 SHV hash. "
fi

[amcmanus]

#106