sim: Add test support for large write alignment

To start adding support for devices with a large write size, modify the
simulator to allow a 'large-write' feature that will also run tests with
128 and 512 byte write alignments.

This requires fixing some places in image generation that assumed a
fairly small alignment (namely, padding the buffer at the end, rather
than individual parts), and detecting the large writes when writing the
MAGIC value to the image.

The flash API still passes the alignment back as a uint_8, but in
overwrite mode, this verifies that the boot code won't use this value.

Currently the 'large-write' feature requires 'overwrite-only' as well.

Signed-off-by: David Brown <david.brown@linaro.org>