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/imp.h

@@ -19,6 +19,9 @@
 #ifndef FLASH_NAND_IMP_H
 #define FLASH_NAND_IMP_H
 
+#include "core.h"
+#include "driver.h"
+
 int nand_write_page(struct nand_device *nand,
 		uint32_t page, uint8_t *data, uint32_t data_size,
 		uint8_t *oob, uint32_t oob_size);