devel/linux-kernel: update to 4.7.9

Changes: https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.7.9
Changes: https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.7.8
Changes: https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.7.7
PR: 213660
Submitted by: Tobias Kortkamp (maintainer)