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/drivers/dfu/dfu_mtd.c b/drivers/dfu/dfu_mtd.c
index 894b570..a31d480 100644
--- a/drivers/dfu/dfu_mtd.c
+++ b/drivers/dfu/dfu_mtd.c
@@ -268,9 +268,9 @@
 	st = strsep(&s, " ");
 	if (!strcmp(st, "raw")) {
 		dfu->layout = DFU_RAW_ADDR;
-		dfu->data.mtd.start = simple_strtoul(s, &s, 16);
+		dfu->data.mtd.start = hextoul(s, &s);
 		s++;
-		dfu->data.mtd.size = simple_strtoul(s, &s, 16);
+		dfu->data.mtd.size = hextoul(s, &s);
 	} else if ((!strcmp(st, "part")) || (!strcmp(st, "partubi"))) {
 		char mtd_id[32];
 		struct mtd_device *mtd_dev;
diff --git a/drivers/dfu/dfu_nand.c b/drivers/dfu/dfu_nand.c
index b8d24d2..6ddf80b 100644
--- a/drivers/dfu/dfu_nand.c
+++ b/drivers/dfu/dfu_nand.c
@@ -204,9 +204,9 @@
 	st = strsep(&s, " ");
 	if (!strcmp(st, "raw")) {
 		dfu->layout = DFU_RAW_ADDR;
-		dfu->data.nand.start = simple_strtoul(s, &s, 16);
+		dfu->data.nand.start = hextoul(s, &s);
 		s++;
-		dfu->data.nand.size = simple_strtoul(s, &s, 16);
+		dfu->data.nand.size = hextoul(s, &s);
 	} else if ((!strcmp(st, "part")) || (!strcmp(st, "partubi"))) {
 		char mtd_id[32];
 		struct mtd_device *mtd_dev;
diff --git a/drivers/dfu/dfu_ram.c b/drivers/dfu/dfu_ram.c
index ab0ce9e..cc7e45b 100644
--- a/drivers/dfu/dfu_ram.c
+++ b/drivers/dfu/dfu_ram.c
@@ -74,8 +74,8 @@
 	}
 
 	dfu->layout = DFU_RAM_ADDR;
-	dfu->data.ram.start = simple_strtoul(argv[1], NULL, 16);
-	dfu->data.ram.size = simple_strtoul(argv[2], NULL, 16);
+	dfu->data.ram.start = hextoul(argv[1], NULL);
+	dfu->data.ram.size = hextoul(argv[2], NULL);
 
 	dfu->write_medium = dfu_write_medium_ram;
 	dfu->get_medium_size = dfu_get_medium_size_ram;
diff --git a/drivers/dfu/dfu_sf.c b/drivers/dfu/dfu_sf.c
index 8f8c425..ef52319 100644
--- a/drivers/dfu/dfu_sf.c
+++ b/drivers/dfu/dfu_sf.c
@@ -171,9 +171,9 @@
 	st = strsep(&s, " ");
 	if (!strcmp(st, "raw")) {
 		dfu->layout = DFU_RAW_ADDR;
-		dfu->data.sf.start = simple_strtoul(s, &s, 16);
+		dfu->data.sf.start = hextoul(s, &s);
 		s++;
-		dfu->data.sf.size = simple_strtoul(s, &s, 16);
+		dfu->data.sf.size = hextoul(s, &s);
 	} else if (CONFIG_IS_ENABLED(DFU_SF_PART) &&
 		   (!strcmp(st, "part") || !strcmp(st, "partubi"))) {
 		char mtd_id[32];