kernel: bump 4.9 to 4.9.206

Refreshed all patches.

Altered patches:
- 400-mtd-add-rootfs-split-support.patch

Compile-tested on: none
Runtime-tested on: none

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

target/linux/generic/pending-4.9/404-mtd-add-more-helper-functions.patch

@@ -11,7 +11,7 @@ Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
 
 --- a/drivers/mtd/mtdpart.c
 +++ b/drivers/mtd/mtdpart.c
-@@ -1222,6 +1222,24 @@ int mtd_is_partition(const struct mtd_in
+@@ -1247,6 +1247,24 @@ int mtd_is_partition(const struct mtd_in
  }
  EXPORT_SYMBOL_GPL(mtd_is_partition);