Move project-specific networking tool paths to those projects

[fhunleth]

Currently vintage_net's application environment contains the paths to wpa_supplicant, pppd, and other tools. It doesn't directly use them and it seems more appropriate for those paths to be moved to vintage_net_wifi, etc.

Move these out and update the docs to indicate that not only do you need to ensure that the base tool set (udhpc, udhcpd, ifconfig, ip) need to be available, but also whatever the vintage_net_* packages you use require.