Fix and improve the generation of the stripped kernel source tree

Fix and improve the generation of the stripped kernel source tree

  * Commit f3757573 broke the rules used for the generation of the
    stripped kernel source tree because of the requirement of the
    rpm directory to be present for CONFIG_KERNEL
    (which is clearly not required for us).

  * Add a new rule to sed Makefile.am and manually set "SUBDIRS" to
    "module include" for CONFIG_KERNEL and remove it for the other
    configs.

    * This has the benefit that now running ./configure --with-config=user
    && make in the DKMS source tree is a nop while before it gave
    an error.

  * Improve the robustness of the rules by ensuring that they will
    abort if something is not as expected.