Since commit fe66b5d2 ("kbuild: refactor kernel-devel RPM package
and linux-headers Deb package"), the kernel-devel RPM package and
linux-headers Deb package are broken.

I double-quoted the $(find ... -type d), which resulted in newlines
being included in the argument to the outer find comment.

  find: 'arch/arm64/include\narch/arm64/kvm/hyp/include': No such file or directory

The outer find command is unneeded.

Fixes: fe66b5d2 ("kbuild: refactor kernel-devel RPM package and linux-headers Deb package")
Reported-by: Karolis M <k4rolis@protonmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>

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>