vbautomation takes shared folder name without prefix + improve timing

Use VBoxControl to check for available shared folders.
By default the VBoxService starts up but it takes some time until
the automount folder is available, so give it some further time.

autoconfig.functions

@@ -2268,33 +2268,58 @@ if $VIRTUALBOX ; then
       VBoxService >/dev/null
       eend $?
 
-      local vbox_automation='/media/sf_automation'
+      local vbautomation='automation'
       if checkbootparam 'vbautomation'; then
-        vbox_automation="$(getbootparam 'vbautomation' 2>>$DEBUG)"
+        vbautomation="$(getbootparam 'vbautomation' 2>>$DEBUG)"
       fi
 
-      local distri="$(getbootparam 'distri' 2>>$DEBUG)"
-      [ -n "$distri" ] || distri='grml'
-
-      if [ -d "${vbox_automation}" ] ; then
-        einfo "Found shared folders automation directory $vbox_automation"
+      if ! VBoxControl sharedfolder list | egrep -q "^[0-9]+ - ${vbautomation}$" ; then
+        ewarn "No automount shared folder '$vbautomation' available"
+        eend 0
+      else
+        einfo "Found automount shared folder '$vbautomation'"
         eend 0
 
+        local distri="$(getbootparam 'distri' 2>>$DEBUG)"
+        [ -n "$distri" ] || distri='grml'
+
+        local vbox_auto_sf="/media/sf_${vbautomation}"
+
+        sleep 1 # ugly but necessary
+
+        counter=10
         eindent
-        if checkbootparam 'novbautomation' ; then
-          einfo "Bootoption novbautomation found. Disabling automation script execution."
+        while ! [ -d "${vbox_auto_sf}" ] && [[ "$counter" != 0 ]]; do
+          einfo "Waiting another second to retry access to ${vbox_auto_sf}"
+          sleep 1
+          counter=$(( counter-1 ))
           eend 0
+        done
+        eoutdent
+
+        if ! [ -d "${vbox_auto_sf}" ] ; then
+          eerror "Giving up trying to access folder ${vbox_auto_sf} which doesn't seem to exist"
+          eend 1
         else
-          if ! [ -x "${vbox_automation}/${distri}" ] ; then
-            ewarn "Couldn't find an automation script named ${vbox_automation}/${distri}"
-            eend 1
+          einfo "Found shared folders automation directory $vbox_auto_sf"
+          eend 0
+
+          eindent
+          if checkbootparam 'novbautomation' ; then
+            einfo "Bootoption novbautomation found. Disabling automation script execution."
+            eend 0
           else
-            einfo "Executing '${vbox_automation}/${distri}' now:"
-            "${vbox_automation}/${distri}"
-            eend $?
+            if ! [ -x "${vbox_auto_sf}/${distri}" ] ; then
+              ewarn "Couldn't find an automation script named ${vbox_auto_sf}/${distri}"
+              eend 1
+            else
+              einfo "Executing '${vbox_auto_sf}/${distri}' now:"
+              "${vbox_auto_sf}/${distri}"
+              eend $?
+            fi
           fi
+          eoutdent
         fi
-        eoutdent
       fi
 
     eoutdent