linux-headers: rework hash exclusion

linux-headers: rework hash exclusion

Although we currently don't have a .hash file for linux-headers, there
already are exclusions for the BR2_KERNEL_HEADERS_AS_KERNEL case (copied
from linux.mk).  However, there is no exclusion for the
BR2_KERNEL_HEADERS_VERSION case.

For the BR2_KERNEL_HEADERS_AS_KERNEL case, the exclusion is actually
not needed. Indeed, KERNEL_HEADERS_SOURCE is computed to be the same
value as LINUX_SOURCE, and linux.mk already adds LINUX_SOURCE to
BR_NO_CHECK_HASH_FOR.

For the other cases, we should exclude the BR2_KERNEL_HEADERS_VERSION
case because there the user supplies the version so it can't be
included in the .hash file.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>