Internal documentation

Constrains:
While mounting, the driver sends over SPI without MMC CS selected, this could
make problems with the mp3 decoder.
Therefore, mount may only be called while there is no mp3 playback.
As everything else is realized by polling, there can't be other interference.

Mount speed improvements:

cluster size: 4K:
  original: 217 sec, clusters 234254 983676
  optimized: 29 sec, clusters 234254 983676

cluster size 16K:
  original: 54 sec, clusters 58625 246280
  optimized: 8 sec, clusters 58625 246280

sdhc readtime: 30sec
free size in MB: 595MB

old mkir timings (16K cluster):
  1. mkdir: 46sec
  2. mkdir: 1sec

Avr assembler output with line numbers:
avr-gcc -S -mmcu=atmega128 -Os -fno-delete-null-pointer-checks -Wall -Wstrict-prototypes -Wa,-ahlms=error.lst -DARTHERNET1 -D__HARVARD_ARCH__ -DPHAT_SECTOR_BUFFERS=4 -DAVR_BUILD -DNUTDEBUG_CHECK_STACK -DNUTDEBUG_HEAP -DNUTMEM_GUARD -I../../ethernut4-8/nut/include -I. -gstabs fs/phat32.c -o fs/phat32.s
