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>
diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index f2b5825..e701429 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -214,7 +214,7 @@
     } scratch;
 
     uint8_t swap_type[BOOT_IMAGE_NUMBER];
-    uint8_t write_sz;
+    uint32_t write_sz;
 
 #if defined(MCUBOOT_ENC_IMAGES)
     struct enc_key_data enc[BOOT_IMAGE_NUMBER][BOOT_NUM_SLOTS];
@@ -229,7 +229,7 @@
                         size_t slen, uint8_t key_id);
 
 int boot_magic_compatible_check(uint8_t tbl_val, uint8_t val);
-uint32_t boot_trailer_sz(uint8_t min_write_sz);
+uint32_t boot_trailer_sz(uint32_t min_write_sz);
 int boot_status_entries(int image_index, const struct flash_area *fap);
 uint32_t boot_status_off(const struct flash_area *fap);
 uint32_t boot_swap_info_off(const struct flash_area *fap);