software/theia: fix frontend-instance-logo errors when updating

When updating existing instance, running slapos node instance failed with:

    ln: failed to create symbolic link '/srv/slapgrid/slappart9/srv/frontend-static/logo.png': File exists

This fixes by doing nothing if logo.png already exists

software/theia/buildout.hash.cfg

@@ -15,7 +15,7 @@
 
 [instance]
 filename = instance.cfg.in
-md5sum = 397fcb3279029af3055b23525d147445
+md5sum = 2ceb9389281c00261abd864fc8ed566f
 
 [yarn.lock]
 filename = yarn.lock

software/theia/instance.cfg.in

@@ -98,8 +98,12 @@ stop-on-error = true
 [frontend-instance-logo]
 recipe = plone.recipe.command
 filename = logo.png
+full-path = $${directory:frontend-static}/$${:filename}
 command =
-  ln -s ${logo.png:output} $${directory:frontend-static}/$${:filename}
+  if [ ! -e $${:full-path} ]
+  then
+    ln -s ${logo.png:output} $${:full-path}
+  fi
 stop-on-error = true
 
 [frontend-instance-slapos.css]