This patch modernizes the 'Block::Session::info' interface. Instead of
using out parameters, the 'init' RPC function returns a compound 'Info'
object now. The rather complicated 'Operations' struct is replaced by
a 'writeable' attribute in the 'Info' object.
Fixes#3275
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