Loop the emergency shell when initial pool imports fail

Originally, ZFSBootMenu would drop to an emergency shell once if the
initial `zpool import -aN` failed to yield a usable pool, then terminate
when the user quit the shell. This required the user attempt to make the
pool importable and then reboot to allow ZFSBootMenu to try again.

With this change, the pool health check and drop to emergency_shell are
looped until a usable pool is identified. This allows the user to
manually import a pool if possible, then type `exit` to allow
ZFSBootMenu to attempt to proceed without an intervening reboot.

Author: ahesford

90zfsbootmenu/zfsbootmenu.sh

@@ -60,20 +60,25 @@ fi
 # Attempt to import all pools read-only
 read_write='' all_pools=yes import_pool
 
+# Make sure at least one pool can be imported; if not,
+# drop to an emergency shell to allow the user to attempt recovery
 import_success=0
-while IFS=$'\t' read -r _pool _health; do
-  [ -n "${_pool}" ] || continue
+while true; do
+  while IFS=$'\t' read -r _pool _health; do
+    [ -n "${_pool}" ] || continue
 
-  import_success=1
-  if [ "${_health}" != "ONLINE" ]; then
-    echo "${_pool}" >> "${BASE}/degraded"
-  fi
-done <<<"$( zpool list -H -o name,health )"
+    import_success=1
+    if [ "${_health}" != "ONLINE" ]; then
+      echo "${_pool}" >> "${BASE}/degraded"
+    fi
+  done <<<"$( zpool list -H -o name,health )"
 
-if [ "${import_success}" -ne 1 ]; then
-  emergency_shell "unable to successfully import a pool"
-  exit
-fi
+  if [ "${import_success}" -ne 1 ]; then
+    emergency_shell "unable to successfully import a pool"
+  else
+    break
+  fi
+done
 
 # Prefer a specific pool when checking for a bootfs value
 # shellcheck disable=SC2154