Reworked file naming and use ROM fs as fall back

The server will now attempt to provide files from the primary
    path RootPath and then the alternate AltRootPath. This alows
    a layered approch to file service.

    Setting the RootPath to an SD card and the AltRootPath to a
    ROM FS will allow a user to provide an updated uavcan firware
    file that will be used instead of the baked in one without
    any copying.

    The path as been removed from the interactions with nodes.
    There by supporting Non-compliant nodes that could not
    handle the specified path length.

    The vitrual pathing has been moved into the FDCache.

libuavcan/include/uavcan/protocol/firmware_update_trigger.hpp

@@ -345,12 +345,7 @@ class FirmwareUpdateTrigger : public INodeInfoListener,
         protocol::file::BeginFirmwareUpdate::Request req;
 
         req.source_node_id = getNode().getNodeID().get();
-        if (!common_path_prefix_.empty())
-        {
-            req.image_file_remote_path.path += common_path_prefix_.c_str();
-            req.image_file_remote_path.path.push_back(protocol::file::Path::SEPARATOR);
-        }
-        req.image_file_remote_path.path += path->c_str();
+        req.image_file_remote_path.path = path->c_str();
 
         UAVCAN_TRACE("FirmwareUpdateTrigger", "Request to %d with path: %s",
                      int(node_id.get()), req.image_file_remote_path.path.c_str());
@@ -396,6 +391,7 @@ public:
      */
     int start(NodeInfoRetriever& node_info_retriever,
               const FirmwareFilePath& arg_common_path_prefix = FirmwareFilePath(),
+              const FirmwareFilePath& arg_alt_path_prefix = FirmwareFilePath(),
               const TransferPriority priority = TransferPriority::OneHigherThanLowest)
     {
         /*