ar71xx: Add kernel 4.9 support

This add support for kernel 4.9 to the ar71xx target.
It was compile tested with the generic, NAND and mikrotik subtarget.
Multiple members of the community tested it on their boards and did not
report any major problem so far.

Especially the NAND part received some changes to adapt to the new
kernel APIs. The serial driver hack used for the Arduino Yun was not
ported because the kernel changed there a lot.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/ar71xx/patches-4.9/420-net-ar71xx_mac_driver.patch

@@ -0,0 +1,28 @@
+--- a/drivers/net/ethernet/atheros/Kconfig
++++ b/drivers/net/ethernet/atheros/Kconfig
+@@ -5,7 +5,7 @@
+ config NET_VENDOR_ATHEROS
+ 	bool "Atheros devices"
+ 	default y
+-	depends on PCI
++	depends on (PCI || ATH79)
+ 	---help---
+ 	  If you have a network (Ethernet) card belonging to this class, say Y.
+ 
+@@ -78,4 +78,6 @@ config ALX
+ 	  To compile this driver as a module, choose M here.  The module
+ 	  will be called alx.
+ 
++source drivers/net/ethernet/atheros/ag71xx/Kconfig
++
+ endif # NET_VENDOR_ATHEROS
+--- a/drivers/net/ethernet/atheros/Makefile
++++ b/drivers/net/ethernet/atheros/Makefile
+@@ -2,6 +2,7 @@
+ # Makefile for the Atheros network device drivers.
+ #
+ 
++obj-$(CONFIG_AG71XX) += ag71xx/
+ obj-$(CONFIG_ATL1) += atlx/
+ obj-$(CONFIG_ATL2) += atlx/
+ obj-$(CONFIG_ATL1E) += atl1e/