nand_spl: display warning message to inform the end of nand_spl

nand_spl: display warning message to inform the end of nand_spl

Now we have two different frameworks for doing the same thing.
I'd like to propose to drop nand_spl framework.
How about deleting it before v2014.07 release?

In the interim, this commit will display a warning message
when compiling nand_spl board.

Now we have 19 nand_spl boards:

  MPC8315ERDB_NAND
  SIMPC8313_LP
  SIMPC8313_SP
  MPC8536DS_NAND
  MPC8569MDS_NAND
  MPC8572DS_NAND
  P1023RDS_NAND
  P1011RDB_NAND
  P1020RDB_NAND
  P2010RDB_NAND
  P2020RDB_NAND
  acadia_nand
  bamboo_nand
  canyonlands_nand
  glacier_nand
  haleakala_nand
  kilauea_nand
  rainier_nand
  sequoia_nand

They must be ported to SPL before the deadline,
otherwise they will be removed.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Dave Liu <daveliu@freescale.com>
Cc: Ron Madrid <info@sheldoninst.com>
Cc: Roy Zang <tie-fei.zang@freescale.com>
Cc: Stefan Roese <sr@denx.de>
Acked-by: Stefan Roese <sr@denx.de>