Merge the platform-specific files and classes (they merely aggregated
themselves) so that each platform provides merely one class
Sd_card::Driver. Also, the Sd_card::Driver_base class is introduced for
the generic parts of Sd_card::Driver.
Ref #2206
Most implementations use a Signal_handler now to acknowledge the packet
instead of waiting for the transfer completion. The exceptions to that are
the non-DMA implementations for RPI and PL180
Ref #2206
