#788 LWIP debug macros

[kaidokert]

I found that lwip debug macros are currently a no-op in trunk. It makes lwip and especially experimental driver debugging somewhat difficult.
Attached is a patch that does two things: adds a lwipPrintf function declaration, that can be inserted by the user into their main program, for example to output to serial, and necessary lwip variable formatters.

Second, implements support for %hu , %hd and %hx 16-bit integer formatters, which are apparently C11 standard now.
Tested on a Olimex P107 rev B - any interest for having fully functional LWIP demo files for this ? Clock config was a bit tricky to get right.