When a GPIO pin is set to be turned on by kernel hints (hint.gpio.X.pinon)
make sure the GPIO pin is configured as an output as this is not always the
case.
Add support for reading RouterBoard's memory which is passed by the loader
(RouterBOOT).

Tested on RouterBoards, various and on RSPRO, TP-Link MR3x20
(for regressions).
Fix a logic bug which prevented the sending of UDP packet with 0 checksum.
This bug was introduced in r264212 and should be X-MFCed with that
revision, if UDP-Lite support if MFCed.