FS: select root hash with root session argument

genode/Tupfile

@@ -1,3 +1,4 @@
 TARGET_NAME = blobsets
 include_rules
 include $(NIMBLE_BINARIES_INCLUDE)
+include $(BIN_RULES)

genode/src/blobsets_fs.nim

@@ -339,11 +339,12 @@ componentConstructHook = proc(env: GenodeEnv) =
               setId = toSetId(e)
               break
             except ValueError: continue
+          if not setId.isNonZero:
+            try: setId = args.argString("root").toSetId
+            except: discard
         if setId.isNonZero:
           try:
-            let
-              #rootPath = args.argString "root"
-              txBufSize = args.argInt "tx_buf_size"
+            let txBufSize = args.argInt "tx_buf_size"
             env.createSession(store, id, label, setId, txBufSize.int)
           except:
             echo "failed to create session for '", label, "', ", getCurrentExceptionMsg()