Allocate DMA buffer for block packet stream

By allocating the packet-stream dataspace for block sessions as
uncached, we can use DMA to directly read and write into the client
buffer. Currently, the OMAP4 SD-card driver is using this feature.

os/include/block/component.h

@@ -189,7 +189,7 @@ namespace Block {
 				}
 
 				return new (md_alloc())
-					Session_component(env()->ram_session()->alloc(tx_buf_size),
+					Session_component(env()->ram_session()->alloc(tx_buf_size, false),
 					                  _driver_factory, _ep);
 			}