[partition] Make mountPoint optional

Some devices cannot be mounted (as DM_verity_hash), therefore, it is
nosense to set a mountPoint for them.

src/modules/partition/core/PartitionLayout.cpp

@@ -134,8 +134,7 @@ PartitionLayout::init( const QVariantList& config )
     {
         QVariantMap pentry = r.toMap();
 
-        if ( !pentry.contains( "name" ) || !pentry.contains( "mountPoint" ) || !pentry.contains( "filesystem" )
-             || !pentry.contains( "size" ) )
+        if ( !pentry.contains( "name" ) || !pentry.contains( "filesystem" ) || !pentry.contains( "size" ) )
         {
             cError() << "Partition layout entry #" << config.indexOf( r )
                      << "lacks mandatory attributes, switching to default layout.";

src/modules/partition/partition.conf

@@ -208,7 +208,7 @@ defaultFileSystemType:  "ext4"
 #   - type: partition type (optional parameter; gpt only; requires KPMCore >= 4.2.0)
 #   - attributes: partition attributes (optional parameter; gpt only; requires KPMCore >= 4.2.0)
 #   - filesystem: filesystem type
-#   - mountPoint: partition mount point
+#   - mountPoint: partition mount point (optional parameter; not mounted if unset)
 #   - size: partition size in bytes (append 'K', 'M' or 'G' for KiB, MiB or GiB)
 #           or
 #           % of the available drive space if a '%' is appended to the value