Add support for waiting for a file/filesystem to appear before activating a layer

[prajnoha]

(This is probably for later, but writing it here so we have that recorded.)

Sometimes, to activate a layer in a device stack, we need an access to a configuration file which in turn resides on a filesystem which is to be mounted (...and another device stack underneath to get activated before that).

Add support in SID for this functionality so that even if we have all the underlying devices in the layer below ready so that the next layer could possibly be activated, we still need to wait for any associated configuration to get accessible so we can actually execute the activation for current layer.