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/tpm-common.c b/cmd/tpm-common.c
index 88c9e08..a48c060 100644
--- a/cmd/tpm-common.c
+++ b/cmd/tpm-common.c
@@ -68,7 +68,7 @@
 	for (i = 0; i < length; i += 2) {
 		byte[0] = bytes[i];
 		byte[1] = bytes[i + 1];
-		data[i / 2] = (u8)simple_strtoul(byte, NULL, 16);
+		data[i / 2] = (u8)hextoul(byte, NULL);
 	}
 
 	if (count_ptr)