backports: split Kconfig into Kconfig.package and Kconfig.sources

backports: split Kconfig into Kconfig.package and Kconfig.sources

This splits up the Kconfig from things which are needed due to
our copy of code from the kernel and things which we need
specifically only for packaging backports. We will later then
share the Kconfig.souce for instance for kernel integration.

While at it, split up the list of files that we need to copy
into what we know could be shared for integration. The the
case of backport packaging Kconfig.package will be copied to
the package's Kconfig when building the package with gentree.py.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>