ar71xx: Ubiquiti Airmax M: add relocate-kernel to invalidate cache

Some Ubiquiti U-boot versions, in particular the "U-Boot 1.1.4.2-s956
(Jun 10 2015 - 10:54:50)" found with AirOS 5.6, do not correctly flush the
caches for the whole kernel address range after decompressing the kernel
image, leading to hard to debug boot failures, depending on kernel version
and configuration.

As a workaround, prepend the relocate-kernels loader, which will invalidate
the caches after moving the kernel to the correct load address.

Reported-by: Andreas Ziegler <dev@andreas-ziegler.de>
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

target/linux/ar71xx/image/generic-ubnt.mk

@@ -52,6 +52,7 @@ define Device/ubnt-xm
   DEVICE_PACKAGES += kmod-usb-ohci
   UBNT_TYPE := XM
   UBNT_CHIP := ar7240
+  KERNEL := kernel-bin | patch-cmdline | relocate-kernel | lzma | uImage lzma
 endef
 
 define Device/ubnt-xw