Update LPC1768 linker script and silence GNU warnings in network stack

I updated the LPC1768 linker script to place AHBSRAM0 and AHBSRAM1 sections of data into the correct regions of memory.  Without these changes, the networking code would try to place all of the lwIP data in the lower 32K and fail to link due to overflow.

I also made a few changes to the networking code to silence various GCC warnings which were emitted at -Wall -Wextra level.  I broke these changes down into separate commits and give more description for each of them in their associated commit descriptions.

Please let me know if there is anything you would like me to modify and I can issue a new pull request with the desired modifications.

Thanks,

Adam