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/bootm_os.c b/common/bootm_os.c
index 0b6325d..d635037 100644
--- a/common/bootm_os.c
+++ b/common/bootm_os.c
@@ -238,7 +238,7 @@
 	/* See README.plan9 */
 	s = env_get("confaddr");
 	if (s != NULL) {
-		char *confaddr = (char *)simple_strtoul(s, NULL, 16);
+		char *confaddr = (char *)hextoul(s, NULL);
 
 		if (argc > 0) {
 			copy_args(confaddr, argc, argv, '\n');
@@ -360,8 +360,7 @@
 			/* find f=0xnumber flag */
 			if ((bootargs[pos] == '=') && (pos >= 1) &&
 			    (bootargs[pos - 1] == 'f')) {
-				vxflags = simple_strtoul(&bootargs[pos + 1],
-							 NULL, 16);
+				vxflags = hextoul(&bootargs[pos + 1], NULL);
 				if (vxflags & VXWORKS_SYSFLG_STD_DTB)
 					std_dtb = true;
 			}