toolchain/common: add minimum kernel headers options

We now have quite a few packages that depend on the kernel headers to be
at least a certain version. For example, dvb-apps requires at least the
3.3 kernel headers.

Add a set of options that packages can depend on, to check that the kernel
headers match their required version.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>