boot: mynewt: Respect minimum write size

For MCUs with restriction on minimum write size (STM32H7)
unaligned writes resulted in flash write errors preventing
any sort of update.

Now MCUBOOT_BOOT_MAX_ALIGN can be set accordingly to value
that flash driver uses.
For alignment <= 8 default value provided by mcuboot config
is still used.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>