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>

boot/mynewt/mcuboot_config/include/mcuboot_config/mcuboot_config.h

@@ -124,6 +124,10 @@
 
 #define MCUBOOT_MAX_IMG_SECTORS       MYNEWT_VAL(BOOTUTIL_MAX_IMG_SECTORS)
 
+#if MYNEWT_VAL(MCU_FLASH_MIN_WRITE_SIZE) > 8
+#define MCUBOOT_BOOT_MAX_ALIGN  MYNEWT_VAL(MCU_FLASH_MIN_WRITE_SIZE)
+#endif
+
 #if MYNEWT_VAL(BOOTUTIL_FEED_WATCHDOG) && MYNEWT_VAL(WATCHDOG_INTERVAL)
 #include <hal/hal_watchdog.h>
 #define MCUBOOT_WATCHDOG_FEED()    \