unionfs: convert to cmake-package

Fixes e.g. http://autobuild.buildroot.net/results/c279007dd8b89a1c6fc9b58d3c2cda01ab84279e



The addition of CPPFLAGS overrides the defines added in unionfs's
Makefile.  Since unionfs now has CMake support, use that instead.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>