kmod: install core module to "extra" subdir

To be more consistent with other out-of-tree modules, install the core
module to /usr[/local]/lib/modules/`uname -r`/extra/kpatch/kpatch.ko.
Merge pull request #183 from jpoimboe/modules-extra

kmod: install core module to "extra" subdir
kpatch-build: check distro in /etc/os-release

/etc/issue isn't valid in RHEL 7 RC.  The official way to query the
distro seems to be /etc/os-release.
Merge pull request #184 from jpoimboe/kb-os-release

kpatch-build: check distro in /etc/os-release
cleanup: remove unneeded patches variable

Signed-off-by: Seth Jennings <sjenning@redhat.com>
merge add-patches-section functionality into create-obj-diff

Signed-off-by: Seth Jennings <sjenning@redhat.com>
rename .patches section to .kpatch.patches

Signed-off-by: Seth Jennings <sjenning@redhat.com>
dynrelas support, obsoleting link-vmlinux-syms

Signed-off-by: Seth Jennings <sjenning@redhat.com>