boot: bootutil: Fix memory alignment of RAM buffer

Change fixes memory alignment of the RAM buffer that is used to
temporarily store data during swap. Some FLASH drivers require
word-aligned input data buffer. Using unaligned buffer results
in FLASH write error.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>
(cherry picked from commit e51ec070)
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>