In the recent dropping of !NET_MULTI code (commit e2a53458),
I misread the logic in include/net.h.  Some of it was used by NET_MULTI
code as glue between the multi/non-multi worlds for cpm2 boards.

Rather than restore the block of code, push the logic to the board config
headers where it all belongs.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>