Read flag is set when the sd controller reads response data in from the
sd card. When the cpu reads from the controller, the flag is reset.
This flag does not trigger an interrupt, it mmust be polled.
The SD card expects data to transition on falling edges and be stable on
rising edges.
Additionally, writes from the CPU were not handled with correct timing.
Now, there is an extra state when writing to the command register so
that the command is properly latched before the CRC is calculated.
Adds the start of the SD card controller which is capable of sending
commands using the SD protocol.
It is accessed by writing the arguments first and triggered by writing
the command number.
