move more nor flash implementation details

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.

src/flash/nor/Makefile.am

@@ -31,6 +31,7 @@ noinst_HEADERS = \
 	at91sam7.h \
 	at91sam3.h \
 	avrf.h \
+	core.h \
 	cfi.h \
 	imp.h \
 	lpc2000.h \