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/load.c b/cmd/load.c
index b7894d7..ec3eed1 100644
--- a/cmd/load.c
+++ b/cmd/load.c
@@ -257,11 +257,11 @@
 #endif
 
 	if (argc >= 2) {
-		offset = simple_strtoul(argv[1], NULL, 16);
+		offset = hextoul(argv[1], NULL);
 	}
 #ifdef	CONFIG_SYS_LOADS_BAUD_CHANGE
 	if (argc >= 3) {
-		size = simple_strtoul(argv[2], NULL, 16);
+		size = hextoul(argv[2], NULL);
 	}
 	if (argc == 4) {
 		save_baudrate = (int)simple_strtoul(argv[3], NULL, 10);
@@ -284,7 +284,7 @@
 	}
 #else	/* ! CONFIG_SYS_LOADS_BAUD_CHANGE */
 	if (argc == 3) {
-		size = simple_strtoul(argv[2], NULL, 16);
+		size = hextoul(argv[2], NULL);
 	}
 #endif	/* CONFIG_SYS_LOADS_BAUD_CHANGE */
 
@@ -438,12 +438,12 @@
 	/* pre-set offset from $loadaddr */
 	s = env_get("loadaddr");
 	if (s)
-		offset = simple_strtoul(s, NULL, 16);
+		offset = hextoul(s, NULL);
 
 	load_baudrate = current_baudrate = gd->baudrate;
 
 	if (argc >= 2) {
-		offset = simple_strtoul(argv[1], NULL, 16);
+		offset = hextoul(argv[1], NULL);
 	}
 	if (argc == 3) {
 		load_baudrate = (int)simple_strtoul(argv[2], NULL, 10);