zfsbootmenu: clean up main loop

Rather than relying on a call to draw_be to determine if the bootenvs
file is empty, the main loop should use the return code of
populate_be_list to determine if any boot environments were found.

The sanity checks in draw_be are still present.

zfsbootmenu/bin/zfsbootmenu

@@ -82,25 +82,19 @@ while true; do
  if [ "${BE_SELECTED}" -eq 0 ]; then
  # Populate the BE list, load any keys as necessary
  # If no BEs were found, remove the empty environment file
- populate_be_list "${BASE}/bootenvs" || rm -f "${BASE}/bootenvs"
+ if ! populate_be_list "${BASE}/bootenvs" ; then
+ rm -f "${BASE}/bootenvs"
 
- bootenv="$( draw_be "${BASE}/bootenvs" )"
- ret=$?
-
- if [ "${ret}" -eq 130 ]; then
- # No BEs were found, so print a warning and drop to the emergency shell
  timed_prompt -d 10 \
  -m "$( colorize red "No boot environments with kernels found" )" \
  -m "$( colorize red "Dropping to an emergency shell to allow recovery attempts" )"
  tput clear
  tput cnorm
  exit 1
- elif [ "${ret}" -ne 0 ]; then
- # Esc was pressed
- continue
  fi
 
- # A selection was made, so split "key,selected_be" pair
+ bootenv="$( draw_be "${BASE}/bootenvs" )" || continue
+
  # shellcheck disable=SC2162
  IFS=, read key selected_be <<<"${bootenv}"
  zdebug "selected key: ${key}"