Make Kernel check not mandatory

Use 2 different checks, one specify a mandatory kernel version
and the other will output a warning and position a preprocessing
variable that can be used in the code to exclude portions of code
that use kernel features not available under a certain version.

Bug-AGL: SPEC-772

Change-Id: I04523abe96749aeeb185e90eee154fd70a4cbf19
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>