extrausers: Use static ids when available

When creating users at the image level using extrausers class, the
current behavior is to ignore the status of USERADDEXTENSION. This could
lead to undefined behavior when static ids are expected but the system
falls back to dynamic ones.

(From OE-Core rev: 331140e892f84b70bced44a0b5d14f32ec95042e)

Signed-off-by: David Vincent <freesilicon@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/extrausers.bbclass

@@ -63,3 +63,7 @@ set_user_group () {
 		remaining=`echo $remaining | cut -d ';' -f2-`
 	done
 }
+
+USERADDEXTENSION ?= ""
+
+inherit ${USERADDEXTENSION}