Adds gokrazy/gafpush and a 'make tsapp-push-pi PI=<ip>' Makefile target for pushing a freshly-built GAF to a running appliance over the network. See the gokrazy/gafpush package doc for details. Updates #1866 Change-Id: Id7a0bad712fcf2eddae593f71d5feacee05c5234 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>