Merged requested to merge add_buildroot_variant into master
add a possibility to use openwrt variants (different directories) with same openwrt revision. This could be used when applying different sets of patches to openwrt that should not be in other openwrt variant. At moment it is not used.
move some ar71xx.generic targets to a new configuration "lowmem" (18.06)
remove all unsued ar71xx targets. If we want to add new devices, we select it in configuration (menuconfig) and until next release/testing it is provided by nightly build