rhel: Add 4.12 kernel support in ovs-kmod-manage.sh

rhel: Add 4.12 kernel support in ovs-kmod-manage.sh

This patch extends c3570519ecaf ("rhel: add 4.4 kernel in kmod build
with mulitple versions, fedora") that updates ovs-kmod-manage.sh to
support SLES 12 SP4 kernel (4.12.x, x>=14).

For some distros, openvswitch-kmod rpm package may contain multiple
ovs kernel modules built against different kernels to deal with kernel
ABI changes and kernel module compatibility issues.  For rpm that
packages with multiple kernel modules, ovs-kmod-manage.sh is invoked
during the rpm post installation stage to 1) select the proper kernel
module to be used; 2) create symbolic links to the proper kernel module
in the weak-updates directory if needed.

For SLES 12 SP4, since the weak-modules utility is not available, even
though there is no ovs related kernel ABI changes for its
currently 5 available kernels from 4.12.14-94.41.1 to 4.12.14-95.16.1,
we still want to invoke ovs-kmod-manage.sh to create weak-updates
symbolic links if the kernel that build the rpm package is different
from the installed kernel.

Notice that ovs-kmod-manage.sh assumes the oldest compatible kernel
is used to build the kernel module rpm. For example, on SLES 12 SP4
it would be,

$ rpmbuild -bb -D 'kversion 4.12.14-94.41-default' \
    rhel/openvswitch-kmod-fedora.spec

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Gurucharan Shetty <guru@ovn.org>