boot: store write align as uint32_t

In overwrite-only mode, the write alignment is not used for much.
Making this a larger value will allow the alignment for a particular
device to be larger than a uint8_t.  This will make the trailer size
very large for these devices (and prevent
!defined(MCUBOOT_OVERWRITE_ONLY) from being used), but at least allows
the overwrite only mode to be used on these devices.

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