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/cmd/bmp.c b/cmd/bmp.c
index f4fe97d..071ba90 100644
--- a/cmd/bmp.c
+++ b/cmd/bmp.c
@@ -131,11 +131,11 @@
 		if (!strcmp(argv[2], "m"))
 			x = BMP_ALIGN_CENTER;
 		else
-			x = simple_strtoul(argv[2], NULL, 10);
+			x = dectoul(argv[2], NULL);
 		if (!strcmp(argv[3], "m"))
 			y = BMP_ALIGN_CENTER;
 		else
-			y = simple_strtoul(argv[3], NULL, 10);
+			y = dectoul(argv[3], NULL);
 		break;
 	default:
 		return CMD_RET_USAGE;