rename nand.h to flash//nand/core.h

rename nand.h to flash//nand/core.h

Rename nand.h as flash/nand/core.h, chase consumers.  The public APIs
need to be sorted out with imp.h, but this allows other changes to
begin improving the separation between policy and mechanism.

Moves #include <target/target.h> and #include "driver.h" into the
internal headers or source files, removing it from <flash/nand/core.h>.