cmd: mtd: add nand_read_test command support

cmd: mtd: add nand_read_test command support

This patch implements read-only test of nand flash devices.

Test reads blocks of NAND flash in normal and raw modes and compares
results. The following statuses can be returned for a block:
 * non-ecc reading failed,
 * ecc reading failed,
 * block is bad,
 * bitflips is above maximum,
 * actual number of biflips above reported one,
 * bitflips reached it maximum value,
 * bitflips above threshold,
 * block is ok.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>