build system: introduce a new feature called build variants.

build system: introduce a new feature called build variants.
it allows building a package multiple times in one pass with different build dirs,
based on the same build settings.
replaces templating hacks like the one used in the hostapd package

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@18357 3c298f89-4303-0410-b956-a3cf2f4a3e73