Merge pull request #217 from grissiom/fix-dfs

DFS: let mount fail on device open error
Merge pull request #214 from grissiom/rtt-next2

Rtt next2
Merge pull request #216 from grissiom/more-tc

More tc
TC: more tolerate on the timeout tick in semaphore_dynamic

With system that has RT_TICK_PER_SECOND is 1000, it's easy to get one
tick error.