Add 'nand verify' command

Add 'nand verify' command

Add the 'nand verify' command to perform a dump and fake-write
simultaneously, checking the read bits against those generated by the
write process.  Appropriate user documentation for this command has
been added to the user guide as well.

The algorithm presently makes a relatively naive comparison.  Some chips
that use ECC may not verify correctly using this implementation, but the
new documentation provides details about this limitation.