NAND read data page refactor.

NAND read data page refactor.

Added a new function to encapsulate reading a page of data from
a NAND device using either the read_block_data function of a NAND
controller or to use direct reading of data from the NAND device.

This also adds some performance enhancements and uses the read_data
function if the read_block_data function fails safely (because it
can't allocate a buffer in the working area).

[dbrownell@users.sourceforge.net: fix fault handling, whitespace]

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>