toolchain/external: check kernel headers version for custom toolchain

Ensure the kernel headers version used in the custom external toolchain,
or the manually-specified kernel headers version, matches exactly the one
selected by the user.

We do not care about the patch-level, since headers are not supposed to
change between patchlevels. This applies only to kernels >= 3.0, but
those are actually the ones we do care about; we treat all 2.6.x kernels
as being a single version, since we do not support any 2.6 kernels for
packages with kernel-dependant features.

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>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>