Support KERNEL_ARCH for AArch64

The architecture tuple is 'aarch64', but the kernel people decided to
call it 'arm64', so we have to do some mungling to get the kernel
architecture name from the Buildroot architecture name.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>