[floppy] get current volume when changing sector's content

Minor, indentation

WIP: refactoring, piu becero fix al problema del bit slip

[dma] read/write code with DMA implemented, now with formatting support Tested with AppleDOS INITIALIZE command and CopyII+ V5. - Writing buffer size increased to keep whole SD Card sector, plus CRC and "end of data token". Buffer is mostly initialized at startup, only address and data fields are changed at runtime. - New writing state machine, which detects the currently writing field and updates the writing buffer accordingly. - Buffer commit is always triggered at the end of a data field. Another write request can now be interleaved while buffer commit is in progress. - When formatting operation is detected (i.e. an address field is written), reading is inhibited.

[dma] first working code with DMA, only read is implemented ALERT: AN HARDWARE CHANGE IS NEEDED, THIS CODE IS COMPATIBLE ONLY WITH HARDWARE REV2.0. - disabled writing, since is currently not implemented through DMA; - removed bitbanging, no more needed; - sdcard SPI is handled with USART in MSPI mode, this need pin swap between MOSI and SCK; - spi speed is incremented to 16MHz. Initialization speed is fixed to 50kHz; - added DMA for floppy sector reading.

[repo] Note in readme

[sdcard] first steps toward rev 2.0 - swapped SDCARD_CLK and SDCARD_DO pins to make use of syncronous UART (MSPI mode). - Switched from SPI to UART, MSPI mode.

[sdcard] Fixed typo in initialization procedure After acmd 41, must wait for empty R1 (0x00), not IDLE (0x01).

[encoder] use in-hardware quadrature encoder system instead of software

[sdcard] faster SPI /4 prescaler instead of /64