Remove extra boards from LIST_ixp

pdnb3 and scpu are explicitly on LIST_ixp, even though they are
also specified in boards.cfg as having cpu ixp. This means that
they will be built twice when doing ./MAKEALL ixp, or ./MAKEALL arm.

This was pointless before, but actually breaks things if you launch
both builds at the same time, as they overwrite each other.

Signed-off-by: Andy Fleming <afleming@freescale.com>

MAKEALL

@@ -365,10 +365,7 @@ LIST_at91="$(boards_by_soc at91)"
 
 LIST_pxa="$(boards_by_cpu pxa)"
 
-LIST_ixp="$(boards_by_cpu ixp)
-	pdnb3		\
-	scpu		\
-"
+LIST_ixp="$(boards_by_cpu ixp)"
 
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 ## ARM groups