luci-app-bmx6: Avoid race condition in bmx6json.lua get()

luci-app-bmx6: Avoid race condition in bmx6json.lua get()

The network topology of a BMX6 mesh can be obtained by asking all the nodes
about their links via the bmx6-info script. The wget() function in the
bmx6json.lua file is part of the process, but it often returns nil, even if
the remote data can actually be fetched, in what seems to be a race condition.
By calling the sys.exec("") function empty, the race condition disappears.

Dirty fix to https://github.com/openwrt-routing/packages/issues/436.

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>