SD card: add Petit FatFs.

Fetched from http://elm-chan.org/fsw/ff/00index_p.html, version R0.03.

Some modifications to avoid file name conflicts with other parts and
to compile code only when needed. Changes are tracked in pff.c.
Move MAX6675 initialisation from mendel.c to temp.c.
Move Intercom temp sensor initialisation from mendel.c to temp.c.
SD card: sd.c heavily reviewed, still not working.
SD card: add the layer between hardware and Petit FatFs.

This is all the commands to read from and write to SPI,
initializing the card, read in blocks and so on. This should
make Petit FatFs actually usable.

So far read-only and no M-codes to let end users play with
this stuff.

Code not tested on actual hardware, but it compiles and was
carefully reviewed visually, so chances are good it mostly
works.
SD card: remove sd.c

This file was never in use and actually didn't even compile.
It didn't disturb so far because its whole content was wrapped
in #if 0. Recently it was fixed to at least compile, but code
quality was found to be poor enough to justify looking for
something written more carefully, which was found in FatFs
sample code.