The kernel-devel RPM package and the linux-headers Debian package
provide headers and scripts needed for building external modules.

They copy the necessary files in slightly different ways - the RPM
copies almost everything except some exclude patterns, while the Debian
copies less number of files. There is no need to maintain different code
to do the same thing.

Split the Debian code out to scripts/package/install-extmod-build, which
is called from both of the packages.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>