Boot: Change the return type of flash_area_align()

Change from uint8_t to uint32_t to prevent an implicit cast. Update
variables and functions which use the return value of the function.

Change-Id: I55bcb1cf2b4b642d1cdf8a1d41c04e601289dea0
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/bl2/ext/mcuboot/flash_map.c b/bl2/ext/mcuboot/flash_map.c
index 5cbb824..2d865a7 100644
--- a/bl2/ext/mcuboot/flash_map.c
+++ b/bl2/ext/mcuboot/flash_map.c
@@ -257,7 +257,7 @@
     return rc;
 }
 
-uint8_t flash_area_align(const struct flash_area *area)
+uint32_t flash_area_align(const struct flash_area *area)
 {
     ARM_FLASH_INFO *flash_info;