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/blk_common.c b/cmd/blk_common.c
index 87f94b5..0898798 100644
--- a/cmd/blk_common.c
+++ b/cmd/blk_common.c
@@ -63,9 +63,9 @@
 
 	default: /* at least 4 args */
 		if (strcmp(argv[1], "read") == 0) {
-			ulong addr = simple_strtoul(argv[2], NULL, 16);
-			lbaint_t blk = simple_strtoul(argv[3], NULL, 16);
-			ulong cnt = simple_strtoul(argv[4], NULL, 16);
+			ulong addr = hextoul(argv[2], NULL);
+			lbaint_t blk = hextoul(argv[3], NULL);
+			ulong cnt = hextoul(argv[4], NULL);
 			ulong n;
 
 			printf("\n%s read: device %d block # "LBAFU", count %lu ... ",
@@ -78,9 +78,9 @@
 			       n == cnt ? "OK" : "ERROR");
 			return n == cnt ? 0 : 1;
 		} else if (strcmp(argv[1], "write") == 0) {
-			ulong addr = simple_strtoul(argv[2], NULL, 16);
-			lbaint_t blk = simple_strtoul(argv[3], NULL, 16);
-			ulong cnt = simple_strtoul(argv[4], NULL, 16);
+			ulong addr = hextoul(argv[2], NULL);
+			lbaint_t blk = hextoul(argv[3], NULL);
+			ulong cnt = hextoul(argv[4], NULL);
 			ulong n;
 
 			printf("\n%s write: device %d block # "LBAFU", count %lu ... ",