add flash/nor/core.[ch]

The newly moved flash TCL routines access the internals of the module
too much.  Fix the layering issues by adding new core NOR flash APIs:

<flash/nor/core.h>:
  - flash_driver_find_by_name() - self-descriptive

<flash/nor/imp.h>:
  - flash_bank_add()            - encapsulates adding banks to bank list
  - flash_bank_list()           - encapsulates retreiving bank list

This allows the externs in flash/nor/imp.h to be removed, and
these mechanisms may now be re-used by other flash module code.

src/flash/nor/imp.h

@@ -19,9 +19,18 @@
 #ifndef FLASH_NOR_IMP_H
 #define FLASH_NOR_IMP_H
 
-#include <flash/flash.h>
+// this is an internal header
+#include "core.h"
 
-extern struct flash_driver *flash_drivers[];
-extern struct flash_bank *flash_banks;
+/**
+ * Adds a new NOR bank to the global list of banks.
+ * @params bank The bank that should be added.
+ */
+void flash_bank_add(struct flash_bank *bank);
+
+/**
+ * @return The first bank in the global list.
+ */
+struct flash_bank *flash_bank_list(void);
 
 #endif // FLASH_NOR_IMP_H