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/mx3.c

@@ -35,7 +35,9 @@ get_next_halfword_from_sram_buffer() not tested
 #include "config.h"
 #endif
 
+#include "imp.h"
 #include "mx3.h"
+#include <target/target.h>
 
 static const char target_not_halted_err_msg[] =
 	"target must be halted to use mx3 NAND flash controller";