buildroot

infra: add to luarocks support for top-level parallel make

The host-luarock dependency is not always satisfied for the extract
phase because the %-extract target is not anymore in the dependency
chain.
To be sure that the dependency is satisfied add the dependency to the
stamp file $(%_TARGET_EXTRACT) instead of the %-extract target.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Francois Perrad <fperrad@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>