most: apply conditional backport patch for kernels older than 4.13

most: apply conditional backport patch for kernels older than 4.13

This patch makes the recipe apply a backport patch on systems
running a kernel older than 4.13.
It is needed to avoid build errors due to API changes.

v2 (jsmoeller): rewrite as KERNEL_VERSION is not available at parsing stage
                This requires quite some plumbing and the maxkver (and minkver) should
                probably go upstream.
                For now we draft this up here as a bbappend.

Change-Id: I48f75f6e98c7b6b4e92b6e43bc3743a5ec3fbbf7
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>