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/setexpr.c b/cmd/setexpr.c
index 1eb67e2..05d6558 100644
--- a/cmd/setexpr.c
+++ b/cmd/setexpr.c
@@ -49,7 +49,7 @@
 		int len;
 		char *str;
 
-		addr = simple_strtoul(&s[1], NULL, 16);
+		addr = hextoul(&s[1], NULL);
 		switch (w) {
 		case 1:
 			p = map_sysmem(addr, sizeof(uchar));
@@ -94,7 +94,7 @@
 	} else {
 		if (w == CMD_DATA_SIZE_STR)
 			return -EINVAL;
-		arg.ival = simple_strtoul(s, NULL, 16);
+		arg.ival = hextoul(s, NULL);
 	}
 	*argp = arg;