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>.

src/flash/nand/core.c

@@ -24,8 +24,6 @@
 #include "config.h"
 #endif
 
-#include <flash/nand.h>
-#include <flash/common.h>
 #include "imp.h"
 
 /* configured NAND devices and NAND Flash command handler */