Splits the exec mode commands out of flash.c into the flash/nor/ files.
The routines used by these high-level commands are moved into nor/core.c,
with their internal declarations placed in nor/imp.h.
Fixes distribution of <flash/nor/core.h> header.
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.
