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>