buildroot

configs: beagleboardx15: fixate the header series

The kernel headers version used to be default when the default was
still 4.9. Now, however, we switched the default headers version to
4.10 but the beagleboardx15 kernel is still 4.9, so we have to set
the kernel headers version explicitly.

Fixes https://gitlab.com/buildroot.org/buildroot/builds/11700987

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>