sdbfs/kernel: major restructuring of the code

This a major thing, that changes a lot. However, now directories work
properly and the code is generally cleaner. This is what I get,
for example, for the wr-nic release gateware on a SPEC:

  spusa.root# find /mnt
  /mnt
  /mnt/WB4-Crossbar-GSI
  /mnt/WR-CORE
  /mnt/WR-CORE/WB4-Crossbar-GSI
  /mnt/WR-CORE/WB4-BlockRAM
  /mnt/WR-CORE/WB4-Bridge-GSI
  /mnt/WR-CORE/WB4-Bridge-GSI/WB4-Crossbar-GSI
  /mnt/WR-CORE/WB4-Bridge-GSI/WR-Mini-NIC
  /mnt/WR-CORE/WB4-Bridge-GSI/WR-Endpoint
  /mnt/WR-CORE/WB4-Bridge-GSI/WR-Soft-PLL
  /mnt/WR-CORE/WB4-Bridge-GSI/WR-PPS-Generator
  /mnt/WR-CORE/WB4-Bridge-GSI/WR-Periph-Syscon
  /mnt/WR-CORE/WB4-Bridge-GSI/WR-Periph-UART
  /mnt/WR-CORE/WB4-Bridge-GSI/WR-Periph-1Wire
  /mnt/WR-CORE/WB4-Bridge-GSI/WB4-Streaming-DMA_0
  /mnt/WR-NIC
  /mnt/WB-VIC-Int.Control
  /mnt/WR-TXTSU
  /mnt/WR-DIO-Core
  /mnt/WR-DIO-Core/WB4-Crossbar-GSI
  /mnt/WR-DIO-Core/WR-1Wire-master
  /mnt/WR-DIO-Core/WB-I2C-Master
  /mnt/WR-DIO-Core/WB-GPIO-Port
  /...