Improvements

Replaced stock Arduino Ethernet library with a customized variant, which saves more than 4KB of flash and 128bytes of RAM, making space for new features.
FTP/Ethernet initialization is quicker and no longer causes a "hickup" (which was visible as a brief delay when displaying the boot menu).
Simple user/password option for the FTP. This is a temporary stopgap, so basic protection of the FTP access is possible. This, however, currently requires a custom build of the firmware, since user/password are configured at compile-time (in config.h). Again, this is a temporary stopgap only. User/password will be configurable eventually and also be included in future standard firmware builds.

Fixes

Proper reset of the Ethernet PHY. Fixes occasional issues where the Wiznet module was stuck and unable to establish a physical connection to the network. (The Wiznet module is not connected to the DAN][ controller's hardware reset signal. The stock Ethernet library only triggered a software reset of the Wiznet controller. The Ethernet PHY (which is a separate controller taking care of auto-negotiation etc) was never reset. The customized library now takes care of also resetting the PHY. So the Ethernet adapter is now fully reset - just as if the hardware reset was connected.)