base-files: fix default pre-/postrm for packages with a dot

Instead of stripping everything after the first dot, strip everything
after the last dot.
This fixes pre-/postrm actions for packages with a dot in their name,
like libusb-1.0.

Fixes #19668.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45702 3c298f89-4303-0410-b956-a3cf2f4a3e73