Convert checksum algorithm chooser to switch case

  local expected_checksum=`echo "$2" | tr [A-Z] [a-z]`

  [ -n "$expected_checksum" ] || return 0

  # If the checksum length is 32 chars, assume MD5, otherwise SHA2

  if [ "${#expected_checksum}" -eq 32 ]; then

  case "${#expected_checksum}" in

  32) # MD5

    [ -n "$HAS_MD5_SUPPORT" ] || return 0

    checksum_command="compute_md5"

  fi

    ;;

  esac

  # If the computed checksum is empty, return failure

  local computed_checksum=`echo "$($checksum_command < "$filename")" | tr [A-Z] [a-z]`