Noisily disable the legacy NAND subsystem.

Legacy NAND is marked for feature removal after April 2009 (i.e. this
upcoming release).  There are still several boards that reference it
(though many do so only for disk-on-chip support which has been silently
disabled for a while now).  These boards will now fail to build
with #error, though the code is still there if the user removes #error.

The plan is to remove the code outright in the next release, along with
any board code that refers to it (such as board/esd/common/auto_update.c).

Also, remove the legacy NAND API description from README.nand.

Signed-off-by: Scott Wood <scottwood@freescale.com>