Makefile: generate CMake toolchain-file in $(O)

A CMake toolchain-file makes it easy to develop CMake-based packages
outside of Buildroot. Just give the toolchain-file to CMake via the
-DCMAKE_TOOLCHAIN_FILE=... option.

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>