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/bootutil/src/loader.c b/bl2/ext/mcuboot/bootutil/src/loader.c
index 406257e..abf88df 100644
--- a/bl2/ext/mcuboot/bootutil/src/loader.c
+++ b/bl2/ext/mcuboot/bootutil/src/loader.c
@@ -209,11 +209,11 @@
     return 0;
 }
 
-static uint8_t
+static uint32_t
 boot_write_sz(void)
 {
-    uint8_t elem_sz;
-    uint8_t align;
+    uint32_t elem_sz;
+    uint32_t align;
 
     /* Figure out what size to write update status update as.  The size depends
      * on what the minimum write size is for scratch area, active image slot.
@@ -763,7 +763,7 @@
     int area_id;
     int rc;
     uint8_t buf[BOOT_MAX_ALIGN];
-    uint8_t align;
+    uint32_t align;
     uint8_t erased_val;
 
     /* NOTE: The first sector copied (that is the last sector on slot) contains