Add CMAKETARGETS infrastructure for CMake packages

The CMAKETARGETS infrastructure makes adding CMake-based packages to
Buildroot easy. It uses the same set of variables as the autotools
infrastructure, except for autoreconf and libtool stuff which is not
needed. Usage: just call CMAKETARGETS instead of AUTOTARGETS.

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>