configs: Add CONFIG_MACVTAP=m (arm64 only)

From the requesting issue:

"This option is necessary for having bridge-like networking on KVM VMs
 with the host root filesystem on NFS (since regular bridge networking
 doesn't work in that case),"

See: https://github.com/raspberrypi/linux/issues/4413



Signed-off-by: Phil Elwell <phil@raspberrypi.com>