openwrt_router.tftp_get_file uses tftp-hpa which isn't in OpenWrt packages anymore

[wwahammy]

openwrt_router.tftp_get_file requires the OpenWrt router have tftp-hpa installed on it. This tool is no longer in the package repository for OpenWrt (or if it is, I can't find it). tftp-hpa should be replaced with the busybox tftp tool if possible.

[mattsm]

We could probably just use ssh/scp instead since those are more likely to be included. Better yet try a variety of options and then give up with a warning message.