Signed-off-by: Luka Perkov <luka@openwrt.org>

make defconfig (or feeds update) raised following error for fossil
package on Ubuntu 14.04 LTS with GNU Make 3.81:
    Makefile:47: *** missing separator.  Stop.

To fix this, empty blocks are now defined instead of using undefine
directive which was added in GNU Make 3.82.

Signed-off-by: Jan Čermák <jan.cermak@nic.cz>

Signed-off-by: Luka Perkov <luka@openwrt.org>