Writing to SD card is slow

[mbeckersys]

Due to lack of advanced buffering methods, writing to the SD card has a low throughput at the moment (around 500 kByte/s). This is because setting up a DMA transaction takes a certain amount of time, so we need to write long bursts instead of only 512Bytes every transaction. This needs to be fixed in the implementation of the FAT32 FS.

Probably merge newest changes from Ada FAT FS from AdaCore/Ada_Drivers_Library, which is where we forked from.