kernel version selection uses build machine rather than target

library version: master:
commit 49b2be326267ca3e843b499c41b784cd0ef110a4 (HEAD -> master, origin/master, origin/HEAD)
Merge: 6f3589a e9c70f7
Author: Drew Fustini
Date: Sun Feb 18 02:04:43 2018 -0600

Merge pull request #233 from zsserg/fixed_segfault_in_event_detection

* Fixed SEGFAULT when calling remove_event_detect() from python callback

The library supports two kernel interfaces, and the detection of this is in setup.py:

kernel = platform.release()

if kernel >= '4.1.0':
kernel41 = [('BBBVERSION41', None)]
else:
kernel41 = None

I am cross-compiling. So the kernel version detected is from my build machine. This means I have to use a pre-4.1.0 kernel to build for a pre-4.1.0 target and a 4.1.0+ build kernel to build for a 4.1.0+ target.

The build should provide an option to specify a kernel build directory where 'make prepare' has been run, so that the kernel version the library supports is based on the target kernel version.