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>

arch/sh/lib/zimageboot.c

@@ -37,11 +37,12 @@ int do_sh_zimageboot(struct cmd_tbl *cmdtp, int flag, int argc,
 	}
 
 	if (s0)
-		zboot_entry = (ulong (*)(int, char * const []))simple_strtoul(s0, NULL, 16);
+		zboot_entry = (ulong (*)(int, char * const []))hextoul(s0,
+									NULL);
 
 	/* empty_zero_page */
 	if (s1)
-		param = (unsigned char*)simple_strtoul(s1, NULL, 16);
+		param = (unsigned char *)hextoul(s1, NULL);
 
 	/* Linux kernel command line */
 	cmdline = (char *)param + COMMAND_LINE;