global: Convert simple_strtoul() with decimal to dectoul()

It is a pain to have to specify the value 10 in each call. Add a new
dectoul() function and update the code to use it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/dfu/dfu_mtd.c b/drivers/dfu/dfu_mtd.c
index a31d480..0b7f177 100644
--- a/drivers/dfu/dfu_mtd.c
+++ b/drivers/dfu/dfu_mtd.c
@@ -279,7 +279,7 @@
 
 		dfu->layout = DFU_RAW_ADDR;
 
-		part = simple_strtoul(s, &s, 10);
+		part = dectoul(s, &s);
 
 		sprintf(mtd_id, "%s,%d", devstr, part - 1);
 		printf("using id '%s'\n", mtd_id);