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>.
To be more informative (and consistent with flash and pld trees), change
'device' parameter name to 'nand' in NAND source files. This change
eliminates confusing 'device->device->' instance from the code, and
it simplifies the forthcoming command handler patches.