avrf.c: Use extended addressing for flash > 0x20000

avrf.c: Use extended addressing for flash > 0x20000

The current method used for flash addressing uses 16 bit. Every access
to flash is 16 bit wide. With 16 address bits one can address 0x10000
unique locations á 16 bits thats 0x20000 bytes.
For flashes bigger than that avrs have an extended addressing with more
than 16 address bits. This is now implemented and used for flashs larger
than 0x20000 bytes.

Change-Id: Id8b6337dde3830fb3c56b9042872e040bb67c12d
Signed-off-by: Lars Pöschel <poeschell+openocd@mailbox.org>
Reviewed-on: http://openocd.zylin.com/5502
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>