ar71xx: use kernel 4.9 by default
Kernel 4.9 support was added about 2 weeks ago and we haven't seen any
major regression so far. This patch was not ported to kernel 4.9, this
needs some additional work:
821-serial-core-add-support-for-boot-console-with-arbitr.patch
Signed-off-by: Hauke Mehrtens <[email protected]>