block_session: extend interface with sync() call

To support components, which implement the block session's server side
rpc object, and which doesn't write data to their device backend immediately,
an additional synchronization call is needed. Thereby, clients like for
instance a file system can tell these components, when a synchronization is
required.

Ref #113

os/src/server/part_blk/back_end.cc

@@ -35,7 +35,7 @@ namespace Partition {
 	Partition           *partition(int num) { return (num < MAX_PARTITIONS) ? _part_list[num] : 0; }
 	size_t               blk_size()         { return  _blk_size; }
 	inline unsigned long max_packets()      { return (MAX_PACKET_SIZE / _blk_size); }
-
+	void                 sync()             { _blk.sync(); }
 
 	/**
 	 * Partition table entry format

os/src/server/part_blk/main.cc

@@ -137,6 +137,8 @@ namespace Block {
 				ops->set_operation(Packet_descriptor::WRITE);
 			}
 
+			void sync() { Partition::sync(); }
+
 			Partition::Partition *partition() { return _partition; }
 	};
 

os/src/server/part_blk/part_blk.h

@@ -72,6 +72,11 @@ namespace Partition {
 	 * Returns block size of back end
 	 */
 	Genode::size_t blk_size();
+
+	/**
+	 * Synchronize with backend device
+	 */
+	void sync();
 }
 
 #endif /* _PART_BLK_H_ */

os/src/server/rom_loopdev/main.cc

@@ -173,6 +173,8 @@ namespace Block {
 				*blk_size  = _block_size;
 				ops->set_operation(Block::Packet_descriptor::READ);
 			}
+
+			void sync() {}
 	};