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/Makefile.am

@@ -2,6 +2,7 @@ AM_CPPFLAGS = -I$(top_srcdir)/src
 
 noinst_LTLIBRARIES = libocdflashnor.la
 libocdflashnor_la_SOURCES = \
+	core.c \
 	tcl.c \
 	$(NOR_DRIVERS)