useradd: Use modern join syntax

string.join() is deprecated syntax and doesn't work under python3
so use the modern sytax instead.

(From OE-Core rev: 402d652edf79bcfe1eaf1a3b55ad1177d1e3ee85)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/useradd.bbclass

@@ -181,7 +181,7 @@ def get_all_cmd_params(d, cmd_type):
         if param:
             params.append(param)
 
-    return string.join(params, "; ")
+    return "; ".join(params)
 
 # Adds the preinst script into generated packages
 fakeroot python populate_packages_prepend () {