kirkwood: switch to kernel 4.9

Add patches-4.9, some of them (heavily) rewritten:
  - ea4500 is upstream available, keep only LEDE changes in dts
  - ea3500 is changed to match the structure of the upstream ea4500 dts
  - nsa310s rewritten to include the common dtsi
  - nsa325 is dropped, since already upstream

Signed-off-by: Paul Wassi <p.wassi@gmx.at>
Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>
[refresh kernel config, add on100, use the switchdev based mv88e6171
driver for the linksys boards, keep lede specific rootfs/kernel
partition names for linksys boards, reorder patches]
Signed-off-by: Mathias Kresin <dev@kresin.me>