base-files: update default url template and complete kconfig help

Change the kconfig default for the repository url template to use "%s"
instead of "%T" as well.

Also complete the kconfig help text do cover the recently introduced
manufacturer, product and hwrev options.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43872 3c298f89-4303-0410-b956-a3cf2f4a3e73