Use ath79 in stead of ar71xx

[trickeydan]

ath79 is the successor of ar71xx. It's modernization under the hood, with the main goal to bring the code into a form that is acceptable for Linux upstream, so that all (most) of the whole ar71xx supported devices can be handled by an upstream, unpatched Linux kernel.1)

https://openwrt.org/docs/techref/targets/ath79

AR150 is supported by this target.

[TimStallard]

Looks like it's only supported in snapshots and not in a proper release yet - so if we stick with stable releases for now, best to wait.

I'm hoping to refactor the makefile to be generic to architectures anyway, eg for new nodes, x86 builds etc, so should be an easy change once that's done.