boot: fdt: Fix tmp type in env_get_bootm_size() and rename to low

Change type of 'tmp' variable from phys_size_t to phys_addr_t and
rename it to 'low' to better describe what the variable represents,
which is either the bootm_low address from environment or start of
DRAM address.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/boot/image-board.c b/boot/image-board.c
index e3d6374..13876b7 100644
--- a/boot/image-board.c
+++ b/boot/image-board.c
@@ -125,8 +125,8 @@
 
 phys_size_t env_get_bootm_size(void)
 {
-	phys_size_t tmp, size;
-	phys_addr_t start;
+	phys_addr_t start, low;
+	phys_size_t size;
 	char *s = env_get("bootm_size");
 
 	if (s)
@@ -140,11 +140,11 @@
 
 	s = env_get("bootm_low");
 	if (s)
-		tmp = simple_strtoull(s, NULL, 16);
+		low = simple_strtoull(s, NULL, 16);
 	else
-		tmp = start;
+		low = start;
 
-	return size - (tmp - start);
+	return size - (low - start);
 }
 
 phys_size_t env_get_bootm_mapsize(void)