global: Convert simple_strtoul() with hex to hextoul()

It is a pain to have to specify the value 16 in each call. Add a new
hextoul() function and update the code to use it.

Add a proper comment to simple_strtoul() while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/blob.c b/cmd/blob.c
index 887219c..e2efae7 100644
--- a/cmd/blob.c
+++ b/cmd/blob.c
@@ -70,10 +70,10 @@
 	else
 		return CMD_RET_USAGE;
 
-	src_addr = simple_strtoul(argv[2], NULL, 16);
-	dst_addr = simple_strtoul(argv[3], NULL, 16);
-	len = simple_strtoul(argv[4], NULL, 16);
-	key_addr = simple_strtoul(argv[5], NULL, 16);
+	src_addr = hextoul(argv[2], NULL);
+	dst_addr = hextoul(argv[3], NULL);
+	len = hextoul(argv[4], NULL);
+	key_addr = hextoul(argv[5], NULL);
 
 	km_ptr = (uint8_t *)(uintptr_t)key_addr;
 	src_ptr = (uint8_t *)(uintptr_t)src_addr;