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/board/freescale/common/cmd_esbc_validate.c b/board/freescale/common/cmd_esbc_validate.c
index 638aa3c..6c09626 100644
--- a/board/freescale/common/cmd_esbc_validate.c
+++ b/board/freescale/common/cmd_esbc_validate.c
@@ -40,7 +40,7 @@
 		hash_str = argv[2];
 
 	/* First argument - header address -32/64bit */
-	haddr = (uintptr_t)simple_strtoul(argv[1], NULL, 16);
+	haddr = (uintptr_t)hextoul(argv[1], NULL);
 
 	/* With esbc_validate command, Image address must be
 	 * part of header. So, the function is called