package/linux-headers: do not default to any version for custom headers

package/linux-headers: do not default to any version for custom headers

Currently, for a custom headers version, or for the same headers as the
kernel, wedefault to a "very old" version (i.e. 2.6.x in practice).

However, as Vivien explained, when using the same headers as the kernel,
and the kernel is set to use the default version (aka latest version
known to Buildroot) of the kernel, one would expect the headers are
automatically tracking the latest version. Off course, that expectation
is broken because of the above.

However, whatever version we default to, it will probably not be
correct, whether we default to the latest version or to the "very old"
version.

So, simply drop the specific default version, so the default is now the
latest version.

Note: that has the potential to break existing defconfig files that
relied on the "very old" version to be the default. Well, whatever,
they'll get a build failre quite early, and it is easy to fix.

Reported-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>