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/common/kallsyms.c b/common/kallsyms.c
index ce42a93..13344e6 100644
--- a/common/kallsyms.c
+++ b/common/kallsyms.c
@@ -31,7 +31,7 @@
 	*caddr = 0;
 
 	while (*sym) {
-		sym_addr = simple_strtoul(sym, &esym, 16);
+		sym_addr = hextoul(sym, &esym);
 		sym = esym;
 		if (sym_addr > addr)
 			break;