improve SD access stability

[catfact]

we define routines for low-level sector access via PDCA here: https://github.com/tehn/aleph/blob/master/avr32_lib/src/filesystem.c

and attach them to this third-party code for FAT manipulation: https://github.com/tehn/aleph/tree/master/avr32_lib/src/fat_io_lib

any of this is fair game for improvement.

in particular:

• there is an extra byte-by-byte copy step, should be unnecessary.
• iirc, the AVR32 chipset has a dedicated PDCA peripheral but there's no ASF support for it, so the access functions are blocking and slow. we could roll our own peripheral driver or see if Atmel has provided it by now (doubt it, recent ASF updates have focused on their ARM stuff.)

[electropourlaction]

would like to add a possibility to read files in alphabetic order, instead of timestamp order, to this issue.

[catfact]

hm, i don't think that's the same category. this is about optimizing PDCA access and FAT parsing. it's orthogonal to application-level features like sorting lists of strings.

if you want to add sorting features to BEES (say), they might be called here: https://github.com/tehn/aleph/blob/dev/apps/bees/src/files.c#L443

if you want to add a string-sorting utility to avr32lib, by all means go for it! then apps can sort things when they want.

[electropourlaction]

I was hoping the sorting could be done already when reading from the card, so that I would not have to sort later :)