luarocks: allow to work with host-luainterpreter

when we embed luajit on target,
we use luarocks with host-luajit instead of host-lua,
in order to use the same interpreter on the both side.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>