split flash.h into into flash/nor/*.h

Move the bulk of the flash.h file into flash/nor/core.h, leaving an
empty husk that will be removed in the next patch.

The NOR driver structure is an implementation detail, so move it into
its own private header file <flash/nor/driver.h> along with helper
declaration for finding them by name.

src/flash/nor/Makefile.am

@@ -34,6 +34,7 @@ noinst_HEADERS = \
 	avrf.h \
 	core.h \
 	cfi.h \
+	driver.h \
 	imp.h \
 	lpc2000.h \
 	lpc288x.h \