Even though 4.8 is not released yet, some people may want to build a
system using the 4.8-rc kernel, and point to the kernel sources as the
kernel headers to use for the toolchain.

In order to make this possible, this commit adds support for specifying
4.8 as the kernel headers version, in both the internal and external
toolchain logic.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
[Thomas: remove support for 4.8 headers selection, and rework commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>