[partition] Migrate the defaultFS type

- remove the m_defaultFSType from PartitionLayout, because it is
  set on construction -- which is too early, before the configuration
  has been read.
- make the default FS explicit in the init() calls which pass in
  a configuration; this needs support in the intermediate
  PartitionCoreModule.

src/modules/partition/core/PartitionLayout.h

@@ -83,7 +83,13 @@ public:
     PartitionLayout( const PartitionLayout& layout );
     ~PartitionLayout();
 
-    void init( const QVariantList& config );
+    /** @brief create the configuration from @p config
+     *
+     * @p config is a list of partition entries (in QVariant form,
+     * read from YAML). If no entries are given, then a single
+     * partition is created with the given @p defaultFsType
+     */
+    void init( FileSystem::Type defaultFsType, const QVariantList& config );
     bool addEntry( const PartitionEntry& entry );
 
     /**
@@ -98,7 +104,6 @@ public:
                                           const PartitionRole& role );
 
 private:
-    FileSystem::Type m_defaultFsType;
     QList< PartitionEntry > m_partLayout;
 };