[kernel] Allow blocking on empty channels

This commit adds a new flag, j6_channel_block, and a new flags param to
the channel_receive syscall. When the block flag is specified, the
caller will block waiting for data on the channel if the channel is
empty.

definitions/objects/channel.def

@@ -16,5 +16,6 @@ object channel : object {
 
     method receive [cap:receive] {
         param data buffer [out]
+        param flags uint64
     }
 }