Symlink Installer to the Desktop only on read-only medium.

Fixes part of #9399

data/system/boot/Bootscript

@@ -119,13 +119,13 @@ fi
 # Now ask the user if he wants to run the Installer or continue to the Desktop.
 freshInstallIndicator=/boot/common/settings/fresh_install
 if [ "$isReadOnly" = "yes" -o -e $freshInstallIndicator ]; then
-	# Create Installer link (using the write overlay)
-	ln -sf /boot/system/apps/Installer /boot/home/Desktop/Installer
-
 	/bin/ReadOnlyBootPrompt
 	if [ $? -eq 0 ]; then
 		launchscript $SCRIPTS/Bootscript.cd
 		exit 0 # and return
+	elif [ "$isReadOnly" = "yes" ] ; then
+		# Create Installer link (using the write overlay)
+		ln -sf /boot/system/apps/Installer /boot/home/Desktop/Installer
 	fi
 fi