Now both, the sd_card_drv and sd_card_bench use LIBS=sd_card_drv.
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
The previous platform-specific implementations differed only in minor aspects. Ref #2206
The i.MX6 driver shares most of its code with the i.MX53 driver. Ref #2206
