dumpdfs: rework for DragonFS 2.0 and also fix endianess issues.
dumpdfs: rework for DragonFS 2.0 and also fix endianess issues.

In addition to fixing this for the new DragonFS 2.0 layout, this commit
also actually fixes dumpdfs compilation on PC by adding the SWAPLONG
macro and using it appropriately.

It also adds a simple usage message to help users.

sp1187/libdragon,DragonMinded/libdragon,DragonMinded/libdragon,sp1187/libdragon,DragonMinded/libdragon,sp1187/libdragon