ppc: Remove corenet_ds boards

These boards have been orphaned for some time and are behind on various
DM migrations. Remove them.

Cc: Priyanka Jain <priyanka.jain@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/doc/README.pblimage b/doc/README.pblimage
index 7fdd26b..58202c1 100644
--- a/doc/README.pblimage
+++ b/doc/README.pblimage
@@ -61,7 +61,7 @@
 Board specific configuration file specifications:
 ------------------------------------------------
 1. Configuration files rcw.cfg and pbi.cfg must present in the
-board/freescale/corenet_ds/, rcw.cfg is for RCW, pbi.cfg is for
+board/freescale/<BOARD>/ directory, rcw.cfg is for RCW, pbi.cfg is for
 PBI instructions. File name must not be changed since they are used
 in Makefile.
 2. These files can have empty lines and lines starting with "#" as first