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/tlv_eeprom.c b/cmd/tlv_eeprom.c
index 5099cbd..2b643f9 100644
--- a/cmd/tlv_eeprom.c
+++ b/cmd/tlv_eeprom.c
@@ -751,7 +751,7 @@
 
 	/* Convert string to binary */
 	for (i = 0, p = (char *)string; i < 6; i++) {
-		buf[i] = p ? simple_strtoul(p, &end, 16) : 0;
+		buf[i] = p ? hextoul(p, &end) : 0;
 		if (p)
 			p = (*end) ? end + 1 : end;
 	}