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/flash.c b/cmd/flash.c
index 240871e..bc93e98 100644
--- a/cmd/flash.c
+++ b/cmd/flash.c
@@ -151,7 +151,7 @@
 	char *ep;
 	char len_used; /* indicates if the "start +length" form used */
 
-	*addr_first = simple_strtoul(arg1, &ep, 16);
+	*addr_first = hextoul(arg1, &ep);
 	if (ep == arg1 || *ep != '\0')
 		return -1;
 
@@ -161,7 +161,7 @@
 		++arg2;
 	}
 
-	*addr_last = simple_strtoul(arg2, &ep, 16);
+	*addr_last = hextoul(arg2, &ep);
 	if (ep == arg2 || *ep != '\0')
 		return -1;
 
@@ -287,7 +287,7 @@
 		return 0;
 	}
 
-	bank = simple_strtoul(argv[1], NULL, 16);
+	bank = hextoul(argv[1], NULL);
 	if ((bank < 1) || (bank > CONFIG_SYS_MAX_FLASH_BANKS)) {
 		printf ("Only FLASH Banks # 1 ... # %d supported\n",
 			CONFIG_SYS_MAX_FLASH_BANKS);
@@ -366,7 +366,7 @@
 		return CMD_RET_USAGE;
 
 	if (strcmp(argv[1], "bank") == 0) {
-		bank = simple_strtoul(argv[2], NULL, 16);
+		bank = hextoul(argv[2], NULL);
 		if ((bank < 1) || (bank > CONFIG_SYS_MAX_FLASH_BANKS)) {
 			printf ("Only FLASH Banks # 1 ... # %d supported\n",
 				CONFIG_SYS_MAX_FLASH_BANKS);
@@ -547,7 +547,7 @@
 		return CMD_RET_USAGE;
 
 	if (strcmp(argv[2], "bank") == 0) {
-		bank = simple_strtoul(argv[3], NULL, 16);
+		bank = hextoul(argv[3], NULL);
 		if ((bank < 1) || (bank > CONFIG_SYS_MAX_FLASH_BANKS)) {
 			printf ("Only FLASH Banks # 1 ... # %d supported\n",
 				CONFIG_SYS_MAX_FLASH_BANKS);