pkg-cmake: install CMake files as part of a package

pkg-cmake: install CMake files as part of a package

Currently, the toolchainfile.cmake and Buildroot.cmake files are
installed outside of any package, just triggered by the toolchain
target.

As part of the per-package SDK effort, we are trying to avoid anything
that installs to the global $(HOST_DIR), and this is one of the
remaining files installed in $(HOST_DIR) outside of any package. We
fix this by installing such files as part of the toolchain package
post-install staging hooks.

Yes, a post-install staging hook to install things to $(HOST_DIR) is a
bit weird, but the toolchain infrastructure is made of target packages
only, and they all install a lot of stuff to $(HOST_DIR) already.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>