mk: print target when reporting build complete

mk: print target when reporting build complete

When a target is finished building, it reports just "Build complete".
When building multiple targets simultaneously, e.g.
	make install T=x86_64-native-linuxapp-*
and one target fails, it's not always obvious which of the builds
failed. To help this, we add the actual target that is completed to the
"Build complete" message.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>