2 Kernel checks available (warning or mandatory)

2 Kernel checks available (warning or mandatory)

Use 2 differents 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.

Change-Id: Ifc6848df1a1a448094f5312ea23e6d4837e8ef14
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>