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/board/siemens/taurus/taurus.c b/board/siemens/taurus/taurus.c
index cad16f9..dae064d 100644
--- a/board/siemens/taurus/taurus.c
+++ b/board/siemens/taurus/taurus.c
@@ -394,10 +394,9 @@
 	unsigned long boot_retry = 0;
 	char boot_buf[10];
 
-	upgrade_available = simple_strtoul(env_get("upgrade_available"), NULL,
-					   10);
+	upgrade_available = dectoul(env_get("upgrade_available"), NULL);
 	if (upgrade_available) {
-		boot_retry = simple_strtoul(env_get("boot_retries"), NULL, 10);
+		boot_retry = dectoul(env_get("boot_retries"), NULL);
 		boot_retry++;
 		sprintf(boot_buf, "%lx", boot_retry);
 		env_set("boot_retries", boot_buf);