rhel: support kmod build against mulitple kernel versions, fedora

rhel: support kmod build against mulitple kernel versions, fedora

This patch ports changes from kmod rhel6 spec file to fedora spec file,
to support packaging kernel modules built against multiple versions of
kernel sources.

RHEL 7.4 introduced backward incompatible changes in the kernel. As
a result, prebuilt PRM packages against kernels newer than 693.17.1
will cannot be used on systems with older kernels, vice versa.

Intended to work only on RHEL 7.4 (kernel version 3.10.0-693.yy.zz).
This patch allows multiple kernel version numbers delimited by
whitespace to be passed as variable "kversion". The result RPM packages
the kernel module .ko files from all specified kernel versions. For
example,

make rpm-fedora-kmod \
    RPMBUILD_OPT='-D "kversion 3.10.0-693.1.1.el7.x86_64 \
    3.10.0-693.17.1.el7.x86_64"'

By default, make tries to build against the current running kernel.

This patch also includes a script to update the weak-update symlinks
if the system kernel version is upgraded or downgraded after
openvswitch-kmod is installed.

Signed-off-by: Martin Xu <martinxu9.ovs@gmail.com>
CC: Greg Rose <gvrose8192@gmail.com>
CC: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Flavio Leitner <fbl@redhat.com>