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/md5sum.c b/cmd/md5sum.c
index 5ae3ddf..0f0e1d3 100644
--- a/cmd/md5sum.c
+++ b/cmd/md5sum.c
@@ -25,7 +25,7 @@
 	if (*dest == '*') {
 		u8 *ptr;
 
-		ptr = (u8 *)simple_strtoul(dest + 1, NULL, 16);
+		ptr = (u8 *)hextoul(dest + 1, NULL);
 		for (i = 0; i < 16; i++)
 			*ptr++ = sum[i];
 	} else {
@@ -46,7 +46,7 @@
 	if (*verify_str == '*') {
 		u8 *ptr;
 
-		ptr = (u8 *)simple_strtoul(verify_str + 1, NULL, 16);
+		ptr = (u8 *)hextoul(verify_str + 1, NULL);
 		memcpy(vsum, ptr, 16);
 	} else {
 		unsigned int i;
@@ -66,7 +66,7 @@
 
 			*nullp = '\0';
 			*(u8 *)(vsum + i) =
-				simple_strtoul(vsum_str + (i * 2), NULL, 16);
+				hextoul(vsum_str + (i * 2), NULL);
 			*nullp = end;
 		}
 	}
@@ -97,8 +97,8 @@
 			return CMD_RET_USAGE;
 	}
 
-	addr = simple_strtoul(*av++, NULL, 16);
-	len = simple_strtoul(*av++, NULL, 16);
+	addr = hextoul(*av++, NULL);
+	len = hextoul(*av++, NULL);
 
 	buf = map_sysmem(addr, len);
 	md5_wd(buf, len, output, CHUNKSZ_MD5);
@@ -147,8 +147,8 @@
 	if (argc < 3)
 		return CMD_RET_USAGE;
 
-	addr = simple_strtoul(argv[1], NULL, 16);
-	len = simple_strtoul(argv[2], NULL, 16);
+	addr = hextoul(argv[1], NULL);
+	len = hextoul(argv[2], NULL);
 
 	buf = map_sysmem(addr, len);
 	md5_wd(buf, len, output, CHUNKSZ_MD5);