openvpn: split easy-rsa into a separate package, it is no longer bundled with the release tarball

git-svn-id: svn://svn.openwrt.org/openwrt@35498 3c298f89-4303-0410-b956-a3cf2f4a3e73
openvpn: remove the stale PKG_REV variable

git-svn-id: svn://svn.openwrt.org/openwrt@35499 3c298f89-4303-0410-b956-a3cf2f4a3e73