version.mk: add "%s" placeholder for dotted target.subtarget notation

Implement "%s" placeholder that expands to either the target name,
e.g. "ar71xx" if the subtarget is generic or to target.subtarget, e.g.
"ar71xx.nand" is a subtarget is choosen.

Also change the default repository url template to use "%s" instead
of "%T" to reflect the directory structure used by the buildbot systems.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
SVN-Revision: 43871