ath79: D-Link DIR-825 B1 add factory.bin recipe

ath79: D-Link DIR-825 B1 add factory.bin recipe

- Bring back factory.bin image which was missing after porting device to ath79 target
- Use default sysupgrade.bin image recipe
- Adjust max image size according to new firmware partition size after
"ath79: expand rootfs for DIR-825-B1 with unused space (aca8bb5)" changes
- Remove support of upgrading from version 19.07, because partition size changes mentioned above

Signed-off-by: Will Moss <willormos@gmail.com>
(cherry picked from commit a58146d452c50387256d4a616c055ddf3248496f)