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>