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/bedbug.c b/cmd/bedbug.c
index bef617b..549c905 100644
--- a/cmd/bedbug.c
+++ b/cmd/bedbug.c
@@ -75,11 +75,11 @@
 
 	if ((flag & CMD_FLAG_REPEAT) == 0) {
 		/* New command */
-		addr = simple_strtoul (argv[1], NULL, 16);
+		addr = hextoul(argv[1], NULL);
 
 		/* If an extra param is given then it is the length */
 		if (argc > 2)
-			len = simple_strtoul (argv[2], NULL, 16);
+			len = hextoul(argv[2], NULL);
 	}
 
 	/* Run the disassembler */
@@ -114,7 +114,7 @@
 		return CMD_RET_USAGE;
 
 	printf ("\nEnter '.' when done\n");
-	mem_addr = simple_strtoul (argv[1], NULL, 16);
+	mem_addr = hextoul(argv[1], NULL);
 
 	while (1) {
 		putc ('\n');