[POWERPC] bootwrapper: Add TARGET_HAS_ETHn tests to ppcboot.h

U-boots more recent than when ppcboot.h was forked allow the board config
file to enable additional ethernet ports explicitly, rather than
using a hardcoded list of targets.  This allows bootwrapper platform
files to do the same.

Fortunately, nothing after the ethernet addresses is of interest to
cuboot platforms, so the inevitable mismatches won't be too catastrophic.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>