Fixups to network code after recent merges.

Peter's and my changes to LPC1768.ld ended up adding the same AHBSRAM0
and AHBSRAM1 section clauses to the script twice.  I removed one copy.

I also pulled Peter's define of the ETHMEM_SECTION macro up into the
previous nested #if so that the preprocessor wouldn't spit out a
redefined macro warning.

I verified that building the code clean before and after these changes
still results in the same .bin file but now without warnings and/or
duplicate code.