nand util: read/write: accept unaligned length

The underlying code in nand_base.c already supports non-page-aligned reads
and writes, but the block-skipping wrapper code did not.

With block skipping, an unaligned start address is not useful since you
really want to be starting at the beginning of a partition -- or at least
that's where you want to start checking for blocks to skip, but we don't
(yet) support that.  So we still require the start address to be aligned.

An unaligned length, though, is useful for passing $filesize to the
read/write command, and handling it does not complicate block skipping.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Tested-by: Ben Gardiner <bengardiner@nanometrics.ca>