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/driver.c

@@ -23,7 +23,7 @@
 #ifdef HAVE_CONFIG_H
 #include <config.h>
 #endif
-#include <flash/nand.h>
+#include "core.h"
 #include "driver.h"
 
 /* NAND flash controller